Process for producing humic acid-bonded metal foil film current collector

ABSTRACT

The invention provides a process for producing a highly oriented humic acid (HA) film-bonded metal foil current collector, comprising (a) preparing a dispersion of HA or chemically functionalized HA (CHA) dispersed in a liquid medium; (b) depositing the HA or CHA dispersion onto a primary surface of a metal foil to form a wet layer under an orientation-inducing stress; (c) removing the liquid medium to form a dried layer having hexagonal carbon planes and an inter-planar spacing d 002  of 0.4 nm to 1.3; and (d) heat-treating the dried layer at a temperature higher than 80° C. to produce the current collector wherein the film contains inter-connected, merged or thermally reduced HA or CHA sheets that are substantially parallel to each other and are chemically bonded to the metal foil. The film has a thermal conductivity of at least 250 W/mK and an electrical conductivity no less than 500 S/cm.

FIELD OF THE INVENTION

The present invention provides a current collector for a lithium batteryor supercapacitor. The current collector is a metal foil bonded with athin film of highly oriented humic acid or humic acid-derived highlyconducting graphitic film.

BACKGROUND

This patent application is directed at a current collector that workswith an anode electrode (anode active material layer) or a cathodeelectrode (cathode active material layer) of a lithium cell (e.g.lithium-ion cell, lithium-metal cell, or lithium-ion capacitor), asupercapacitor, a non-lithium battery (such as the zinc-air cell, nickelmetal hydride battery, sodium-ion cell, and magnesium-ion cell), andother electrochemical energy storage cells. This application is not partof the anode active material layer or the cathode active material layerper se.

The lithium-metal cell includes the conventional lithium-metalrechargeable cell (e.g. using a lithium foil as the anode and MnO₂particles as the cathode active material), lithium-air cell (Li-Air),lithium-sulfur cell (Li—S), and the emerging lithium-graphene cell(Li-graphene, using graphene sheets as a cathode active material),lithium-carbon nanotube cell (Li—CNT, using CNTs as a cathode), andlithium-nano carbon cell (Li—C, using nano carbon fibers or other nanocarbon materials as a cathode). The anode and/or the cathode activematerial layer can contain some lithium, or can be prelithiated prior toor immediately after cell assembly.

Rechargeable lithium-ion (Li-ion), lithium metal, lithium-sulfur, and Limetal-air batteries are considered promising power sources for electricvehicle (EV), hybrid electric vehicle (HEV), and portable electronicdevices, such as lap-top computers and mobile phones. Lithium as a metalelement has the highest lithium storage capacity (3,861 mAh/g) comparedto any other metal or metal-intercalated compound as an anode activematerial (except Li_(4.4)Si, which has a specific capacity of 4,200mAh/g). Hence, in general, Li metal batteries (having a lithium metalanode) have a significantly higher energy density than conventionallithium-ion batteries (having a graphite anode).

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having relatively high specific capacities, suchas TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathode active materials,which were coupled with a lithium metal anode. When the battery wasdischarged, lithium ions were transferred from the lithium metal anodeto the cathode through the electrolyte and the cathode became lithiated.Unfortunately, upon repeated charges and discharges, the lithium metalresulted in the formation of dendrites at the anode that ultimatelycaused internal shorting, thermal runaway, and explosion. As a result ofa series of accidents associated with this problem, the production ofthese types of secondary batteries was stopped in the early 1990'sgiving ways to lithium-ion batteries. Even now, cycling stability andsafety concerns remain the primary factors preventing the furthercommercialization of Li metal batteries (e.g. Lithium-sulfur andLithium-transition metal oxide cells) for EV, HEV, and microelectronicdevice applications.

Prompted by the aforementioned concerns over the safety of earlierlithium metal secondary batteries led to the development of lithium-ionsecondary batteries, in which pure lithium metal sheet or film wasreplaced by carbonaceous materials (e.g. natural graphite particles) asthe anode active material. The carbonaceous material absorbs lithium(through intercalation of lithium ions or atoms between graphene planes,for instance) and desorbs lithium ions during the re-charge anddischarge phases, respectively, of the lithium-ion battery operation.The carbonaceous material may comprise primarily graphite that can beintercalated with lithium and the resulting graphite intercalationcompound may be expressed as Li_(x)C₆, where x is typically less than 1(with graphite specific capacity <372 mAh/g).

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost, safety, and performance targets (such as highspecific energy, high energy density, good cycle stability, and longcycle life). Li-ion cells typically use a lithium transition-metal oxideor phosphate as a positive electrode (cathode) that de/re-intercalatesLi⁺ at a high potential with respect to the carbon negative electrode(anode). The specific capacity of lithium transition-metal oxide orphosphate based cathode active material is typically in the range of140-170 mAh/g. As a result, the specific energy (gravimetric energydensity) of commercially available Li-ion cells featuring a graphiteanode and a lithium transition-metal oxide or phosphate based cathode istypically in the range of 120-220 Wh/kg, most typically 150-200 Wh/kg.The corresponding typical range of energy density (volumetric energydensity) is from 400 to 550 Wh/L. The energy densities are even lowerunder high charge-discharge rate conditions. These specific energyvalues are two to three times lower than what would be required ifbattery-powered electric vehicles are to be widely accepted.

A typical battery cell is composed of (a) an anode current collector,(b) an anode electrode (also referred to as the anode active materiallayer, typically including an anode active material, a conductivefiller, and a binder resin component) bonded to the anode currentcollector with a binder resin, (c) an electrolyte/separator, (d) acathode electrode (also referred to as the cathode active materiallayer, typically including a cathode active material, a conductivefiller, and a binder resin), (e) a cathode current collector bonded tothe cathode electrode with a binder resin, (f) metal tabs that areconnected to external wiring, and (g) casing that wraps around all othercomponents except for the tabs.

Current collectors, typically aluminum foil (at the cathode) and copperfoil (at the anode), account for about 15-20% by weight and 10-15% bycost of a lithium-ion battery. Therefore, thinner, lighter foils wouldbe preferred. However, there are several major issues associated withstate-of-the-art current collectors: (a) Due to easy creasing andtearing, thinner foils tend to be more expensive and harder to workwith; (b) Current collectors must be electrochemically stable withrespect to the cell components over the operating potential window ofthe electrode. In practice, continued corrosion of the currentcollectors mainly by the electrolyte can lead to a gradual increase inthe internal resistance of the battery, resulting in persistent loss ofthe apparent capacity or poor cycling life: (c) Oxidation of metalcurrent collectors is a strong exothermic reaction that cansignificantly contribute to thermal runaway of a lithium battery.

Accordingly, the current collectors are crucially important for cost,weight, safety, and performance of a battery. Instead of metals,graphene or graphene-coated solid metal or plastic has been consideredas a potential current collector material, as summarized in thereferences listed below:

-   1. Li Wang, Xiangming He, Jianjun Li, Jian Gao, Mou Fang, Guangyu    Tian, Jianlong Wang, Shoushan Fan, “Graphene-coated plastic film as    current collector for lithium/sulfur batteries,” J. Power Source,    239 (2013) 623-627.-   2. S. J. Richard Prabakar, Yun-Hwa Hwang, Eun Gyoung Bae, Dong Kyu    Lee, Myoungho Pyo, “Graphene oxide as a corrosion inhibitor for the    aluminum current collector in lithium ion batteries,” Carbon,    52 (2013) 128-136.-   3. Yang Li, et al. Chinese Patent Pub. No. CN 104600320 A (2015 May    6).-   4. Zhaoping Liu, et al (Ningbo Institute of Materials and Energy,    China), WO 2012/151880 A1 (Nov. 15, 2012).-   5. Gwon, H.; Kim, H-S; Lee, K E; Seo, D-H; Park, Y C; Lee, Y-S; Ahn,    B T; Kang, K “Flexible energy storage devices based on graphene    paper,” Energy and Environmental Science. 4 (2011) 1277-1283.-   6. Ramesh C. Bhardwaj and Richard M. Mank, “Graphene current    collectors in batteries for portable electronic devices,” US    20130095389 A1, Apr. 18, 2013.

Currently, graphene current collectors come in three different forms:graphene-coated substrate [Ref. 1-4], free-standing graphene paper [Ref.5], and monolayer graphene film produced by transition metal (Ni,Cu)-catalyzed chemical vapor deposition (CVD) followed by metal etching[Ref. 6].

In the preparation of graphene-coated substrate, small isolated sheetsor platelets of graphene oxide (GO) or reduced graphene oxide (RGO) arespray-deposited onto a solid substrate (e.g. plastic film or Al foil).In the graphene layer, the building blocks are separated graphenesheets/platelets (typically 0.5-5 μm in length/width and 0.34-30 nm inthickness) that are typically bonded by a binder resin, such as PVDF[Refs. 1, 3, and 4]. Although individual graphene sheets/platelets canhave a relatively high electrical conductivity (within the confine ofthat 0.5-5 μm), the resulting graphene-binder resin composite layer isrelatively poor in electrical conductivity (typically <100 S/cm and moretypically <10 S/cm). Furthermore, another purpose of using a binderresin is to bond the graphene-binder composite layer to the substrate(e.g. Cu foil); this implies that there is a binder resin (adhesive)layer between Cu foil and the graphene-binder composite layer.Unfortunately, this binder resin layer is electrically insulating andthe resulting detrimental effect seems to have been totally overlookedby prior workers.

Although Prabakar, et al. [Ref 2] does not seem to have used a binderresin in forming an aluminum foil coated with discrete graphene oxidesheets, this graphene oxide-coated Al foil has its own problem. It iswell-known in the art that aluminum oxide (Al₂O₃) readily forms onsurfaces of an aluminum foil and cleaning with acetone or alcohol is notcapable of removing this passivating layer of aluminum oxide or alumina.This aluminum oxide layer is not only electrically and thermallyinsulating, but actually is not resistant to certain types ofelectrolyte. For instance, the most commonly used lithium-ion batteryelectrolyte is LiPF₆ dissolved in an organic solvent. A trace amount ofH₂O in this electrolyte can trigger a series of chemical reactions thatinvolve formation of HF (a highly corrosive acid) that readily breaks upthe aluminum oxide layer and continues to corrode the Al foil andconsume electrolyte. The capacity decay typically becomes much apparentafter 200-300 charge-discharge cycles.

Free-standing graphene paper is typically prepared by vacuum-assistedfiltration of GO or RGO sheets/platelets suspended in water. In afree-standing paper, the building blocks are separated graphenesheets/platelets that are loosely overlapped together. Again, althoughindividual graphene sheets/platelets can have a relatively highelectrical conductivity (within the confine of that 0.5-5 μm), theresulting graphene paper has a very low electrical conductivity; e.g.8,000 S/m or 80 S/cm [Ref. 5], which is 4 orders of magnitude lower thanthe conductivity of Cu foil (8×10⁵ S/cm).

There are several major problems associated with the most commonly usedprocess for producing graphene (i.e. the chemicaloxidation/intercalation process):

-   -   (1) The process requires the use of large quantities of several        undesirable chemicals, such as sulfuric acid, nitric acid, and        potassium permanganate or/and sodium chlorate.    -   (2) The thermal exfoliation requires a high temperature        (typically 800-1,050° C.) and, hence, is a highly        energy-intensive process.    -   (3) The approach requires a very tedious washing and        purification step. For instance, typically 2.5 kg of water is        used to wash and recover 1 gram of GIC, producing huge        quantities of waste water that need to be properly treated.    -   (4) The resulting products are graphene oxide (GO) platelets        that must undergo a further chemical reduction treatment to        reduce the oxygen content. Typically even after reduction, the        electrical conductivity of GO platelets remains much lower than        that of pristine graphene. Furthermore, the reduction procedure        often involves the utilization of toxic chemicals, such as        hydrazine.    -   (5) Furthermore, the quantity of intercalation solution retained        on the flakes after draining may range from 20 to 150 parts of        solution by weight per 100 parts by weight of graphite flakes        (pph) and more typically about 50 to 120 pph. During the        high-temperature exfoliation, the residual intercalate species        retained by the flakes decompose to produce various species of        sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which        are undesirable. The effluents require expensive remediation        procedures in order not to have an adverse environmental impact.

The catalyzed CVD process for graphene production involves introductionof a hydrocarbon gas into a vacuum chamber at a temperature of 500-800°C. Under these stringent conditions, the hydrocarbon gas gets decomposedwith the decomposition reaction being catalyzed by the transition metalsubstrate (Ni or Cu). The Cu/Ni substrate is then chemically etched awayusing a strong acid, which is not an environmentally benign procedure.The whole process is slow, tedious, and energy-intensive, and theresulting graphene is typically a single layer graphene or few-layergraphene (up to 5 layers maximum since the underlying Cu/Ni layer losesits effectiveness as a catalyst).

Bhardwaj, et al [Ref. 6] suggested stacking multiple CVD-graphene filmsto a thickness of 1 μm or a few μm; however, this would require hundredsor thousands of films stacked together (each film being typically 0.34nm to 2 nm thick). Although Bhardwaj, et al claimed that “The graphenemay reduce the manufacturing cost and/or increase the energy density ofa battery cell,” no experimental data was presented to support theirclaim. Contrary to this claim, the CVD graphene is a notoriouslyexpensive process and even a single-layer of CVD graphene film would besignificantly more expensive than a sheet of Cu or Al foil given thesame area (e.g. the same 5 cm×5 cm). A stack of hundreds or thousands ofmono-layer or few-layer graphene films as suggested by Bhardwaj, et alwould mean hundreds or thousands times more expensive than a Cu foilcurrent collector. This cost would be prohibitively high. Further, thehigh contact resistance between hundreds of CVD graphene films in astack and the relatively low conductivity of CVD graphene would lead toan overall high internal resistance, nullifying any potential benefit ofusing thinner films (1 μm of graphene stack vs. 10 μm of Cu foil) toreduce the overall cell weight and volume. It seems that the patentapplication of Bhardwaj, et al [Ref. 6], containing no data whatsoever,is nothing but a concept paper.

The above discussions have clearly shown that all three forms of thegraphene-enhanced or graphene-based current collector do not meet theperformance and cost requirements for use in a battery orsupercapacitor. A strong need exists for a different type of materialfor use as a current collector.

The present invention is directed at a new class of materials, hereinreferred to as a highly oriented film of humic acid (HA), alone or incombination with graphene, which is chemically bonded to metal foilsurface. Graphene used herein includes pristine graphene, grapheneoxide, graphene fluoride, nitrogenated graphene, hydrogenated graphene,boron-doped graphene, any other type of doped graphene, and other typeof chemically functionalized graphene. Quite unexpectedly andsignificantly, this highly oriented film of HA or HA/graphene mixturecan be thermally converted to a highly conducting graphitic film.

Humic acid (HA) is an organic matter commonly found in soil and can beextracted from the soil using a base (e.g. KOH). HA can also beextracted, with a high yield, from a type of coal called leonardite,which is a highly oxidized version of lignite coal. HA extracted fromleonardite contains a number of oxygenated groups (e.g. carboxyl groups)located around the edges of the graphene-like molecular center (SP² coreof hexagonal carbon structure). This material is slightly similar tographene oxide (GO) which is produced by strong acid oxidation ofnatural graphite. HA has a typical oxygen content of 5% to 42% by weight(other major elements being carbon and hydrogen). HA, after chemical orthermal reduction, has an oxygen content of 0.01% to 5% by weight. Forclaim definition purposes in the instant application, humic acid (HA)refers to the entire oxygen content range, from 0.01% to 42% by weight.The reduced humic acid (RHA) is a special type of HA that has an oxygencontent of 0.01% to 5% by weight.

It is surprising to discover that humic acid, when brought in intimatecontact with a surface of a metal foil, can chemically bond to the metalfoil. It is further surprising to discover that, when properly alignedand packed together, humic acid molecules can chemically link with oneanother to obtain longer and wider humic acid sheets. These humic acidmolecules are also capable of chemically linking or bonding withgraphene sheets, if present and properly aligned and packed. Theresulting humic acid- or graphitic film-bonded thin metal foil iselectrolyte-compatible, non-reactive, corrosion-protective, of lowcontact resistance, thermally and electrically conductive, ultra-thin,and light-weight, enabling a battery or capacitor to deliver a higheroutput voltage, higher energy density, high rate-capability, and muchlonger cycle life.

SUMMARY OF THE INVENTION

The present invention provides a highly oriented humic acid-bonded metalfoil current collector for use in a battery or supercapacitor. Theinvention also provides a current collector composed of a metal foil anda humic acid-derived highly conducting graphitic film bonded to one ortwo primary surfaces of the metal foil. The invention also providesprocesses for producing these current collectors.

The invented current collector comprises: (a) a thin metal foil having athickness from 1 μm to 30 μm (preferably from 4 μm to 12 μm) and twoopposed but substantially parallel primary surfaces; and (b) at leastone thin film of highly oriented humic acid (HA) or a mixture of HA andgraphene sheets (or a highly conducting graphitic film derived from thisthin film) being chemically bonded to at least one of the two opposedprimary surfaces of the metal foil. The thin film of HA or HA/graphenemixture or the derived graphitic film has a thickness from 10 nm to 10μm, an oxygen content from 0.01% to 10% by weight, a physical densityfrom 1.3 to 2.2 g/cm³, hexagonal carbon planes being orientedsubstantially parallel to each other and parallel to the primarysurfaces, an inter-planar spacing of 0.335 to 0.50 nm between hexagonalcarbon planes, a thermal conductivity greater than 250 W/mK (moretypically >500 W/mK), and an electrical conductivity greater than 800S/cm (more typically >1,500 S/cm) when measured alone without said thinmetal foil.

Preferably, each of the two opposed primary surfaces is chemicallybonded with such a thin film of humic acid or HA/graphene mixture or agraphitic film derived from this thin film produced through heattreatments. Also preferably, one or both thin films of HA or both HA andgraphene (or the derived graphitic film) are chemically bonded to one orboth opposed primary surfaces of the metal foil without using a binderor adhesive. If a binder is used, this binder is an electricallyconductive material selected from an intrinsically conductive polymer,pitch, amorphous carbon, or carbonized resin (polymeric carbon).Preferably, the thin metal foil has a thickness from 4 to 12 μm. Alsopreferably, the thin film of humic acid or HA/graphene mixture or thegraphitic film has a thickness from 20 nm to 2 μm.

For the current collector, preferably the metal foil is selected fromCu, Ti, Ni, stainless steel, Al foil, or a combination thereof.Preferably, the primary surface does not contain a layer of passivatingmetal oxide thereon (e.g. no alumina, Al₂O₃, on Al foil surface).

Preferably, the thin film of HA or HA/graphene mixture or the graphiticfilm derived therefrom has an oxygen content from 1% to 5% by weight.Further preferably, the thin film or the graphitic film derivedtherefrom has an oxygen content less than 1%, an inter-planar spacingless than 0.345 nm, and an electrical conductivity no less than 3,000S/cm. More preferably, the thin film or the graphitic film derivedtherefrom has an oxygen content less than 0.1%, an inter-planar spacingless than 0.337 nm, and an electrical conductivity no less than 5,000S/cm. Still more preferably, the thin film or the graphitic film derivedtherefrom has an oxygen content no greater than 0.05%, an inter-planarspacing less than 0.336 nm, a mosaic spread value no greater than 0.7,and an electrical conductivity no less than 8,000 S/cm. Even morepreferably, the thin film or the graphitic film derived therefrom has aninter-planar spacing less than 0.336 nm, a mosaic spread value nogreater than 0.4, and an electrical conductivity greater than 10,000S/cm.

More preferably, the thin film of HA or HA/graphene mixture or thegraphitic film derived therefrom exhibits an inter-planar spacing lessthan 0.337 nm and a mosaic spread value less than 1.0. Most preferably,the thin film e or the graphitic film derived therefrom exhibits adegree of graphitization no less than 80% and/or a mosaic spread valueno greater than 0.4.

In certain embodiments, the thin film of HA or HA/graphene mixture isobtained by depositing a suspension of HA or a mixture of HA andgraphene sheets onto said at least one primary surface under theinfluence of an orientation-controlling stress to form a layer of HA ora mixture of HA and graphene sheets and then heat-treating said layer ata heat treatment temperature from 80° C. to 1,500° C. More preferably,the heat treatment temperature is from 80° C. to 500° C. and furthermore preferably from 80° C. to 200° C.

The highly oriented thin film of HA or HA/graphene or the graphitic filmderived therefrom bonded to the underlying current collector typicallycontains chemically bonded humic acid molecules or chemically mergedhumic acid and graphene planes that are parallel to one another.Preferably, the thin film is a continuous length film having a length noless than 5 cm and a width no less than 1 cm and this thin film is madeby a roll-to-roll process.

Preferably, the thin film of HA or HA/graphene mixture or the graphiticfilm derived therefrom, when measured alone (as a free-standing layerwithout the presence of a metal foil), has a physical density greaterthan 1.6 g/cm3, and/or a tensile strength greater than 30 MPa. Morepreferably, the thin film or the graphitic film derived therefrom, whenmeasured alone, has a physical density greater than 1.8 g/cm3, and/or atensile strength greater than 50 MPa. Most preferably, the thin film orthe graphitic film derived therefrom, when measured alone, has aphysical density greater than 2.0 g/cm³, and/or a tensile strengthgreater than 80 MPa.

The present invention also provides a rechargeable lithium battery orlithium-ion battery containing the presently invented current collectoras an anode current collector and/or a cathode current collector. Therechargeable lithium battery may be a lithium-sulfur cell, alithium-selenium cell, a lithium sulfur/selenium cell, a lithium-aircell, a lithium-graphene cell, or a lithium-carbon cell.

The present invention also provides a capacitor containing the inventedcurrent collector as an anode current collector or a cathode currentcollector, which capacitor is a symmetric ultracapacitor, an asymmetricultracapacitor cell, a hybrid supercapacitor-battery cell, or alithium-ion capacitor cell.

The invention also provides a process for producing a highly orientedhumic acid film-bonded metal foil current collector for use in a batteryor supercapacitor. The process comprises:

-   (a) preparing a dispersion of humic acid (HA) or chemically    functionalized humic acid (CHA) sheets dispersed in a liquid medium,    wherein the HA sheets contain an oxygen content higher than 5% by    weight or the CHA sheets contain non-carbon element content higher    than 5% by weight;-   (b) dispensing and depositing the HA or CHA dispersion onto at least    one primary surface of a metal foil to form a wet layer of HA or CHA    on the surface, wherein the dispensing and depositing procedure    includes subjecting the dispersion to an orientation-inducing    stress;-   (c) partially or completely removing the liquid medium from the wet    layer of HA or CHA to form a dried HA or CHA layer having hexagonal    carbon planes and an inter-planar spacing d₀₀₂ of 0.4 nm to 1.3 nm    as determined by X-ray diffraction; and-   (d) heat-treating the dried HA or CHA layer at a first heat    treatment temperature higher than 80° C. for a sufficient period of    time to produce the highly oriented humic acid film-bonded metal    foil current collector wherein the humic acid film contains    inter-connected, merged or thermally reduced HA or CHA sheets that    are substantially parallel to each other and are chemically bonded    and parallel to the primary surface and the humic acid film has a    physical density no less than 1.3 g/cm³, a thermal conductivity of    at least 250 W/mK, and/or an electrical conductivity no less than    500 S/cm. The process may further comprise a step of compressing the    humic acid film of merged or reduced HA or CHA after said step (d).

The process may comprise an additional step (e) of further heat-treatingthe humic acid film-bonded metal foil at a second heat treatmenttemperature higher than the first heat treatment temperature for asufficient period of time to produce a graphitic film-bonded metal foilcurrent collector, wherein the graphitic film has an inter-planarspacing d₀₀₂ less than 0.4 nm and an oxygen content or non-carbonelement content less than 5% by weight; and (f) compressing thegraphitic film to produce a highly conducting graphitic film having aphysical density no less than 1.3 g/cm³, a thermal conductivity of atleast 500 W/mK, and/or an electrical conductivity no less than 1,000S/cm. The highly conductive graphitic film preferably has a thicknessfrom 5 nm to 20 μm, but more preferably from 10 nm to 2 μm.

The HA or CHA dispersion may further contain graphene sheets ormolecules dispersed therein and the HA-to-graphene or CHA-to-grapheneratio is from 1/100 to 100/1 wherein the graphene is selected frompristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene bromide, graphene iodide, boron-doped graphene,nitrogen-doped graphene, chemically functionalized graphene, or acombination thereof. The process may include additional step (e) offurther heat-treating the humic acid film of merged or reduced HA or CHAat a second heat treatment temperature higher than the first heattreatment temperature for a sufficient period of time to produce agraphitic film having an inter-planar spacing d₀₀₂ less than 0.4 nm andan oxygen content or non-carbon element content less than 5% by weight;and step (f) of compressing the graphitic film to produce a highlyconducting graphitic film having a physical density no less than 1.6g/cm³, a thermal conductivity of at least 700 W/mK, and/or an electricalconductivity no less than 1,500 S/cm.

In certain embodiments, the HA or CHA sheets are in an amount sufficientto form a liquid crystal phase in the liquid medium. Preferably, thedispersion contains a first volume fraction of HA or CHA dispersed inthe liquid medium that exceeds a critical volume fraction (V_(c)) for aliquid crystal phase formation and the dispersion is concentrated toreach a second volume fraction of HA or CHA, greater than the firstvolume fraction, to improve a HA or CHA sheet orientation. Preferably,the first volume fraction is equivalent to a weight fraction of from0.05% to 3.0% by weight of HA or CHA in the dispersion. The dispersionmay be concentrated to contain higher than 3.0% but less than 15% byweight of HA or CHA dispersed in the liquid medium prior to said step(b).

In some embodiments, the dispersion further contains a polymer dissolvedin said liquid medium or attached to HA or CHA.

The CHA may contain a chemical functional group selected from a polymer,SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′,SR′, SiR′₃, Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X,TlZ₂ and Mg—X; wherein y is an integer equal to or less than 3, R′ ishydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, orpoly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl,fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate ortrifluoroacetate, or a combination thereof.

The graphene sheets, if present, may contain chemically functionalizedgraphene containing a chemical functional group selected from a polymer,SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′,SR′, SiR′₃, Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X,TlZ₂ and Mg—X; wherein y is an integer equal to or less than 3, R′ ishydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, orpoly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl,fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate ortrifluoroacetate, or a combination thereof.

Preferably, the liquid medium consists of water or a mixture of waterand an alcohol. Alternatively, the liquid medium contains a non-aqueoussolvent selected from polyethylene glycol, ethylene glycol, propyleneglycol, an alcohol, a sugar alcohol, a polyglycerol, a glycol ether, anamine based solvent, an amide based solvent, an alkylene carbonate, anorganic acid, or an inorganic acid.

The second heat treatment temperature may be higher than 1,500° C. for alength of time sufficient for decreasing an inter-plane spacing d₀₀₂ toa value less than 0.36 nm and decreasing the oxygen content ornon-carbon element content to less than 0.1% by weight. Specifically,the second heat treatment temperature may be from 1,500° C. to 3,200° C.

The process is preferably a roll-to-roll or reel-to-reel process,wherein step (b) includes feeding a sheet of the metal foil from aroller to a deposition zone, depositing a layer of HA or CHA dispersiononto at least one primary surface of the metal foil to form a wet layerof HA or CHA dispersion thereon, drying the HA or CHA dispersion to forma dried HA or CHA layer deposited on metal foil surface, and collectingthe HA or CHA layer-deposited metal foil on a collector roller.

In certain embodiments, the first heat treatment temperature contains atemperature in the range of 100° C.-1,500° C. and the highly orientedhumic acid film has an oxygen content less than 2.0%, an inter-planarspacing less than 0.35 nm, a physical density no less than 1.6 g/cm³, athermal conductivity of at least 800 W/mK, and/or an electricalconductivity no less than 2,500 S/cm. In other embodiments, the firstheat treatment temperature contains a temperature in the range of 1,500°C.-2,100° C. and the highly oriented humic acid film, becoming a highlyconducting graphitic film, has an oxygen content less than 1.0%, aninter-planar spacing less than 0.345 nm, a thermal conductivity of atleast 1,000 W/mK, and/or an electrical conductivity no less than 5,000S/cm.

In some embodiments, the first and/or second heat treatment temperaturecontains a temperature greater than 2,100° C. and the highly conductinggraphitic film has an oxygen content no greater than 0.1%, aninter-graphene spacing less than 0.340 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 1,300 W/mK, and/oran electrical conductivity no less than 8,000 S/cm. If the second heattreatment temperature contains a temperature no less than 2,500° C., thehighly conducting graphitic film has an inter-graphene spacing less than0.336 nm, a mosaic spread value no greater than 0.4, a thermalconductivity greater than 1,500 W/mK, and/or an electrical conductivitygreater than 10,000 S/cm. The degree of graphitization may be no lessthan 80% and a mosaic spread value less than 0.4.

Typically, the HA or CHA sheets have a maximum original length and thehighly oriented humic acid film contains HA or CHA sheets having alength larger than the maximum original length. This implies that somehumic acid molecules have merged with other HA molecules in anedge-to-edge manner to increase the length or width of the planarmolecules or sheets. The step (e) of heat-treating induces chemicallinking, merging, or chemical bonding of HA or CHA sheets with other HAor CHA sheets, or with graphene sheets to form a graphitic structure.The highly conducting graphitic film is a poly-crystal graphenestructure having a preferred crystalline orientation as determined bysaid X-ray diffraction method.

The process typically results in the formation of a highly orientedgraphitic film having an electrical conductivity greater than 5,000S/cm, a thermal conductivity greater than 800 W/mK, a physical densitygreater than 1.9 g/cm³, a tensile strength greater than 80 MPa, and/oran elastic modulus greater than 60 GPa. Further typically, the highlyoriented graphitic film has an electrical conductivity greater than8,000 S/cm, a thermal conductivity greater than 1,200 W/mK, a physicaldensity greater than 2.0 g/cm³, a tensile strength greater than 100 MPa,and/or an elastic modulus greater than 80 GPa. With a final heattreatment temperature (the first or second heat treatment temperature)higher than 1,500° C., the highly oriented graphitic film has anelectrical conductivity greater than 12,000 S/cm, a thermal conductivitygreater than 1,500 W/mK, a physical density greater than 2.1 g/cm³, atensile strength greater than 120 MPa, and/or an elastic modulus greaterthan 120 GPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) A flow chart illustrating various prior art processes forproducing exfoliated graphite products (flexible graphite foils andflexible graphite composites) and pyrolytic graphite (bottom portion),along with a process for producing isolated graphene sheets andaggregates of graphene or graphene oxide sheets in the form of agraphene paper or membrane.

FIG. 1(B) The prior art graphene-coated metal foil current collectorwith a binder resin layer between the graphene layer (a graphene-resincomposite) and the metal foil (e.g. Cu foil).

FIG. 1(C) A free-standing, non-supported thin metal foil with oneprimary surface of the metal foil being bonded with a thin film of HA orHA/graphene mixture.

FIG. 2 An SEM image of a cross-section of a flexible graphite foil,showing many graphite flakes with orientations not parallel to theflexible graphite foil surface plane and also showing many defects,kinked or folded flakes.

FIG. 3(A) A SEM image of a HA liquid crystal-derived HOGF, whereinmultiple hexagonal carbon planes are seamlessly merged intocontinuous-length graphene-like sheets or layers that can run for tensof centimeters wide or long (only a 50 μm width of a 10-cm wide HOGFbeing shown in this SEM image);

FIG. 3(B) A SEM image of a cross-section of a conventional graphenepaper prepared from discrete reduced graphene oxide sheets/plateletsusing a paper-making process (e.g. vacuum-assisted filtration). Theimage shows many discrete graphene sheets being folded or interrupted(not integrated), with orientations not parallel to the film/papersurface and having many defects or imperfections;

FIG. 3(C) Schematic of a film of highly oriented humic acid moleculesbeing chemically merged together to form a highly ordered and conductinggraphitic film.

FIG. 4(A) Thermal conductivity values of the HA/GO-derived HOGF,GO-derived HOGF, HA-derived HOGF, and FG foil plotted as a function ofthe final heat treatment temperature;

FIG. 4(B) Thermal conductivity values of the HA/GO-derived HOGF,HA-derived HOGF, and polyimide-derived HOPG, all plotted as a functionof the final HTT; and

FIG. 4(C) Electric conductivity values of the HA/GO-derived HOGF,GO-derived HOGF, HA-derived HOGF, and FG foil plotted as a function ofthe final heat treatment temperature.

FIG. 5(A) Inter-graphene plane spacing in HA-derived HOGF measured byX-ray diffraction;

FIG. 5(B) The oxygen content in the HA-derived HOGF;

FIG. 5(C) The correlation between inter-graphene spacing and the oxygencontent; and

FIG. 5(D) Thermal conductivity values of the HA/GO-derived HOGF,GO-derived HOGF, HA-derived HOGF, and FG foil plotted as a function ofthe final heat treatment temperature.

FIG. 6 Thermal conductivity of HOGF samples plotted as a function of theproportion of GO sheets in a HA/GO suspension.

FIG. 7(A) Tensile strength values of HA/GO-derived HOGF, GO-derivedHOGF, HA-derived HOGF, flexible graphite foil, and reduced grapheneoxide paper, all plotted as a function of the final heat treatmenttemperature;

FIG. 7(B) Tensile modulus of the HA/GO-derived HOGF, GO-derived HOGF,and HA-derived HOGF, plotted as a function of the final heat treatmenttemperature.

FIG. 8 Thermal conductivity of three HA-derived highly oriented films;one obtained by heat-treating a HA film that was peeled off from a glasssurface, one deposited on and bonded to Ti surface while beingheat-treated, and one deposited on and bonded to a Cu foil surface whilebeing heat-treated.

FIG. 9(A) The discharge capacity values of three Li—S cells each as afunction of the charge/discharge cycle number; first cell havingHA-bonded Cu foil and HA-bonded Al foil as the anode and cathode currentcollectors, respectively; second cell having GO/resin-coated Cu foil andGO-coated Al foil (no pre-etching) as the anode and cathode currentcollector, respectively (a prior art cell); third cell having a Cu foilanode current collector and Al foil cathode current collector (a priorart cell).

FIG. 9(B) Ragone plots of the three cells: first cell having HA-bondedCu foil and HA-bonded Al foil as the anode and cathode currentcollectors, respectively; second cell having GO/resin-coated Cu foil andGO-coated Al foil (no pre-etching) as the anode and cathode currentcollector, respectively (a prior art cell); third cell having a Cu foilanode current collector and Al foil cathode current collector (a priorart cell).

FIG. 10 The cell capacity values of three magnesium metal cells; firstcell having HA-bonded Cu foil and HA-bonded Al foil as the anode andcathode current collectors, respectively; second cell havingGO/resin-coated Cu foil and GO-coated Al foil (no pre-etching) as theanode and cathode current collector, respectively (a prior art cell);third cell having a Cu foil anode current collector and Al foil cathodecurrent collector (a prior art cell).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a humic acid-bonded metal foil thin-filmcurrent collector (e.g. as schematically shown in FIG. 1(C)) for use ina battery or supercapacitor. In a preferred embodiment, the currentcollector comprises: (a) a free-standing, non-supported thin metal foil(214 in FIG. 1(C)) having a thickness from 1 μm to 30 μm and two opposedbut substantially parallel primary surfaces; and (b) a thin film 212 ofhumic acid (HA) or HA/graphene mixture chemically bonded to at least oneof the two opposed primary surfaces (without using a binder oradhesive). FIG. 1(C) only shows one primary surface of the metal foil214 being bonded with a thin film 212 of HA or HA/graphene mixture.However, preferably, the opposite primary surface is also bonded with athin film of HA or HA/graphene mixture (not shown in FIG. 1(C)). As aterminal pole for electrically connecting the battery/supercapacitor toan external circuit, a metal tab 218 is typically welded or soldered tothe metal foil 214.

As illustrated in FIG. 1(C), a preferred embodiment of the presentinvention is a HA-bonded metal foil current collector, wherein no binderresin layer or passivating aluminum oxide layer is present between thefilm of HA or HA/graphene mixture and the Cu foil or Al foil. Incontrast, as schematically illustrated in FIG. 1(B), the prior artgraphene-coated metal foil current collector typically and necessarilyrequires a binder resin layer between the graphene layer (agraphene-resin composite) and the metal foil (e.g. Cu foil). In the caseof prior art graphene-coated Al foil [Prabakar et al.; Ref 2], apassivating aluminum oxide (alumina) layer is naturally present betweenthe graphene layer and the Al metal foil. This is due to the well-knownfact that aluminum foil, upon fabrication and exposure to room air,always forms a passivating aluminum oxide layer on the surfaces of theAl foil. Simple cleaning by acetone or alcohol is incapable of removingthis alumina layer. As will be demonstrated in later paragraphs, thepresence of a layer of binder resin or aluminum oxide, even as thin asjust 1 nm, has an enormous effect on increasing the contact resistancebetween the graphene layer and the metal foil. This surprising discoveryby us has been totally overlooked by all prior art workers and, hence,prior art graphene-coated metal foils have not met the performance andcost requirements of a lithium battery or supercapacitor currentcollector.

A very significant and unexpected advantage of bringing humic acidsheets in direct contact with the primary surfaces of a Cu, Ni, steel,or Ti foil is the notion that HA molecules can be well-bonded to thesemetal foils under the presently invented processing conditions, withoutusing an external resin binder or adhesive (hence, no dramaticallyincreased contact resistance). These processing conditions includewell-aligning HA (or a mixture of HA and graphene) molecules or sheetson the metal foil surface and then heat-treating the two-layer structureat a temperature in the range of 80° C.-1,500° C. (more typically anddesirably of 80° C.-500° C., and most typically and desirably of 80°C.-200° C.). Optionally, but not preferably, the heat treatmenttemperature can be as high as 1,500-3,000° C. (provided the metal foilcan withstand such a high temperature).

These processing conditions, in the cases of aluminum foil-based currentcollectors, preferably include chemically etching off the passivatingaluminum oxide layer prior to being coated with and bonded by HA,followed by a heat treatment under comparable temperature conditionsdescribed above. Alternatively, the HA molecules may be prepared in anacidic state, which is characterized by having high oxygen contents,reflecting high amounts of —OH and —COOH groups and having a pH valueless than 5.0 (preferably <3.0 and even more preferably <2.0). The Alfoil may be allowed to get immersed in a bath of HA solution, whereinthe acidic environment naturally removes the passivating Al₂O₃ layer.When the Al foil emerges from the bath, HA molecules or sheets naturallyadhere to the clean, etched Al foil surfaces, effectively preventing theexposure of Al foil surfaces to open air (hence, no passivating Al₂O₃layer and no added contact resistance between an Al foil surface and theHA layer). This strategy has never been previously disclosed orsuggested.

In addition to the chemical bonding power of the presently invented HAlayer and the chemical etching power of the HA solution, the resultingthin film of HA or HA/graphene mixture in the presently inventedHA-bonded metal foil has a thickness from 10 nm to 10 μm, an oxygencontent from 0.1% to 10% by weight, an inter-graphene plane spacing of0.335 to 0.50 nm, a physical density from 1.3 to 2.2 g/cm³, all HA andgraphene sheets (if present) being oriented substantially parallel toeach other and parallel to the primary surfaces, exhibiting a thermalconductivity greater than 500 W/mK, and/or electrical conductivitygreater than 1,500 S/cm when measured alone without the thin metal foil.This thin film of HA or HA/graphene is chemically inert and provides ahighly effective protective layer against corrosion of the underlyingmetal foil.

Now, let us take a closer look at the magnitude of the total resistance(including the contact resistance) in a three-layer structure asillustrated in FIG. 1(B). The electrons in the graphene layer 202(Layer 1) must move around in this layer, move across through the binderresin or passivating alumina layer 206 (Layer 2), and then move in themetal foil layer 204 (Layer 3) toward the terminal tab 208. Forsimplicity, we will consider only the total resistance against theelectrons moving across the thickness of the graphene layer, thethickness of the binder/passivating layer, and the thickness of themetal foil layer. The electron movement in both the in-plane directionsof graphene or metal foil is fast and of low resistance; hence, thisresistance is neglected in the instant calculation.

The thickness-direction resistance of a sheet/film of conductor is givenby R=(1/σ) (t/A), where A=cross-section (length×width) of the conductor,t=thickness of the conductor, σ=conductivity=1/

, and

=resistivity, a material constant. A graphene-coated current collectorcontaining a binder or passivating metal oxide layer may be viewed as athree-layer structure (FIG. 1(B)) with the graphene film, interfacialbinder resin layer (or passivating alumina layer), and metal foil layerelectrically connected in series.

The total resistance is the sum of the resistance values of the threelayers:R=R₁+R₂+R₃=ρ₁(t₁/A₁)+ρ₂(t₂/A₂)+ρ₃(t₃/A₃)=(1/σ₁)(t₁/A₁)+(1/σ₂)(t₂/A₂)+(1/σ₃)(t₃/A₃),where ρ=resistivity, σ=conductivity, t=thickness, and A=area of a layer,and, approximately, A₁=A₂=A₃. Scanning electron microscopic examinationsreveal that the binder resin or passivating alumina layers are typically5-100 nm thick. The resistivity of most commonly used binder resin(PVDF) and that of alumina (Al₂O₃) are typically in the range of10¹³-10¹⁵ ohm-cm. Assume that A₁=A₂=A₃=1 cm², the thickness-directionresistivity ρ₁ of graphene layer=0.1 ohm-cm, the binder or alumina layerresistivity ρ₂=1×10¹⁴ ohm-cm and the metal foil layer resistivity isρ₃=1.7×10⁻⁶ ohm-cm (Cu foil), or ρ₃=2.7×10⁻⁶ ohm-cm (Al foil). Alsoassume the optimum conditions where the Cu or Al foil thickness=6 μm,graphene layer thickness=1 μm, and the binder resin layer thickness isonly 0.5 nm (actually it is from 5 nm to 100 nm). Then, the totalresistance of the three-layer structure would be 5×10⁶ ohm and theoverall conductivity would be as low as 1.4×10⁻¹⁰ S/cm (see first datarow in Table 1 below). If we assume that the binder resin layer is 10 nmthick, the total resistance of the three-layer structure would be 1×10⁸ohm and the overall conductivity would be as low as 7.0×10⁻¹² S/cm (see4th data row in Table 1 below). Such a 3-layer composite structure wouldnot be a good current collector for a battery or supercapacitor since ahigh internal resistance would mean a low output voltage and high amountof internal heat generated. Similar results are observed for Ni, Ti, andstainless steel foil-based current collectors (data rows 7-10 of Table1).

TABLE 1 ρ₁ t₁ t₂ t₃ σ = ohm- 10⁻⁴ A₁ ρ₂ 10⁻⁴ A₂ ρ₃ 10⁻⁴ A₃ R t/(AR)metal cm cm cm² ohm-cm cm cm² ohm-cm cm cm² ohm S/cm Cu 0.1 1 1 1.00E+140.0005 1 1.70E−06 6 1 5.00E+06 1.40E−10 Cu 0.1 1 1 1.00E+14 0.001 11.70E−06 6 1 1.00E+07 7.00E−11 Cu 0.1 1 1 1.00E+14 0.005 1 1.70E−06 6 15.00E+07 1.40E−11 Cu 0.1 1 1 1.00E+14 0.01 1 1.70E−06 6 1 1.00E+087.01E−12 Cu 0.1 1 1 1.00E+14 0.001 1 1.70E−06 6 1 1.00E+07 7.00E−11 Al0.1 1 1 1.00E+14 0.001 1 2.70E−06 6 1 1.00E+07 7.00E−11 Ni 0.1 1 11.00E+14 0.001 1 7.00E−06 6 1 1.00E+07 7.00E−11 Ti 0.1 1 1 1.00E+140.001 1 5.50E−05 6 1 1.00E+07 7.00E−11 SS304 0.1 1 1 1.00E+14 0.001 17.20E−05 6 1 1.00E+07 7.00E−11

In contrast, if there is no binder resin or alumina layer (t₂=0), as isthe case of the presently invented current collector, the totalresistance of a graphene oxide-bonded Cu foil has a value of 1.0×10⁻⁵ohm (vs. 1.0×10⁺⁷ ohm of a 3-layer structure containing a 1-μm binderresin layer). Please see Table 2 below. This represents a difference by12 orders of magnitude (not 12-fold)! The conductivity would be 7.0×10⁺¹S/cm for the instant 2-layer structure, in contrast to 7.0×10⁻¹¹ S/cm ofthe corresponding 3-layer structure. Again, the difference is by 12orders of magnitude. Furthermore, we have discovered that the lithiumbatteries and supercapacitors featuring the presently invented grapheneoxide-bonded metal foil current collectors always exhibit a highervoltage output, higher energy density, higher power density, more stablechare-discharge cycling response, and last longer without capacity decayor corrosion issues as compared to prior art graphene-based currentcollectors

TABLE 2 ρ₁ t₁ t₂ t₃ σ = ohm- 10⁻⁴ A₁ ρ₂ 10⁻⁴ A₂ ρ₃ 10⁻⁴ A₃ R t/(AR)metal cm cm cm² ohm-cm cm cm² ohm-cm cm cm² ohm S/cm Cu 0.1 1 1 1.00E+140 1 1.70E−06 6 1 1.00E−05 7.00E+01 Al 0.1 1 1 1.00E+14 0 1 2.70E−06 6 11.00E−05 7.00E+01 Ni 0.1 1 1 1.00E+14 0 1 7.00E−06 6 1 1.00E−05 7.00E+01Ti 0.1 1 1 1.00E+14 0 1 5.50E−05 6 1 1.00E−05 6.98E+01 SS304 0.1 1 11.00E+14 0 1 7.20E−05 6 1 1.00E−05 6.97E+01 Cu 0.1 1 1 1.00E+14 0 11.70E−06 12 1 1.00E−05 1.30E+02 Al 0.1 1 1 1.00E+14 0 1 2.70E−06 12 11.00E−05 1.30E+02 Ni 0.1 1 1 1.00E+14 0 1 7.00E−06 12 1 1.00E−051.30E+02 Ti 0.1 1 1 1.00E+14 0 1 5.50E−05 12 1 1.01E−05 1.29E+02 SS3040.1 1 1 1.00E+14 0 1 7.20E−05 12 1 1.01E−05 1.29E+02 Cu 0.1 5 1 1.00E+140 1 1.70E−06 100 1 5.00E−05 2.10E+02 Al 0.1 5 1 1.00E+14 0 1 2.70E−06100 1 5.00E−05 2.10E+02 Ni 0.1 5 1 1.00E+14 0 1 7.00E−06 100 1 5.01E−052.10E+02 Ti 0.1 5 1 1.00E+14 0 1 5.50E−05 100 1 5.06E−05 2.08E+02 SS3040.1 5 1 1.00E+14 0 1 7.20E−05 100 1 5.07E−05 2.07E+02

In what follows, a description of humic acid and graphene, the two mainingredients in the thin film coated on a metal foil, is presented.

Bulk natural flake graphite is a 3-D graphitic material with eachparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are different in orientation. In other words, the orientations ofthe various grains in a graphite particle typically differ from onegrain to another.

A graphite single crystal (crystallite) per se is anisotropic with aproperty measured along a direction in the basal plane (crystallographica- or b-axis direction) being dramatically different than the propertymeasured along the crystallographic c-axis direction (thicknessdirection). For instance, the thermal conductivity of a graphite singlecrystal can be up to approximately 1,920 W/mK (theoretical) or 1,800W/mK (experimental) in the basal plane (crystallographic a- and b-axisdirections), but that along the crystallographic c-axis direction isless than 10 W/mK (typically less than 5 W/mK). Further, the multiplegrains or crystallites in a graphite particle are typically all orientedalong different directions. Consequently, a natural graphite particlecomposed of multiple grains of different orientations exhibits anaverage property between these two extremes (i.e. typically <100 W/mK).

The constituent graphene planes (typically 30 nm-2 μm wide/long) of agraphite crystallite can be exfoliated and extracted or isolated fromthe graphite crystallite to obtain individual graphene sheets of carbonatoms provided the inter-planar van der Waals forces can be overcome. Anisolated, individual graphene sheet of hexagonal carbon atoms iscommonly referred to as single-layer graphene. A stack of multiplegraphene planes bonded through van der Waals forces in the thicknessdirection with an inter-graphene plane spacing of 0.3354 nm is commonlyreferred to as a multi-layer graphene. A multi-layer graphene platelethas up to 300 layers of graphene planes (<100 nm in thickness), but moretypically up to 30 graphene planes (<10 nm in thickness), even moretypically up to 20 graphene planes (<7 nm in thickness), and mosttypically up to 10 graphene planes (commonly referred to as few-layergraphene in scientific community). Single-layer graphene and multi-layergraphene sheets are collectively called “nano graphene platelets”(NGPs). Graphene sheets/platelets or NGPs are a new class of carbon nanomaterial (a 2-D nano carbon) that is distinct from the 0-D fullerene,the 1-D CNT, and the 3-D graphite.

Our research group pioneered the development of pristine graphenematerials, isolated graphene oxide sheets, and related productionprocesses as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaledGraphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), applicationsubmitted on Oct. 21, 2002; (2) B. Z. Jang, et al. “Process forProducing Nano-scaled Graphene Plates,” U.S. patent application Ser. No.10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo,“Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S.patent application Ser. No. 11/509,424 (Aug. 25, 2006). Historically,Brodie first demonstrated the synthesis of graphite oxide in 1859 byadding a portion of potassium chlorate to a slurry of graphite in fumingnitric acid. In 1898, Staudenmaier improved on this procedure by usingconcentrated sulfuric acid as well as fuming nitric acid and adding thechlorate in multiple aliquots over the course of the reaction. Thissmall change in the procedure made the production of highly oxidizedgraphite in a single reaction vessel significantly more practical. In1958, Hummers reported the method most commonly used today: the graphiteis oxidized by treatment with KMnO₄ and NaNO₃ in concentrated H₂SO₄.However, these earlier work failed to isolate and identify fullyexfoliated and separated graphene oxide sheets. These studies alsofailed to disclose the isolation of pristine, non-oxidized single-layeror multiple-layer graphene sheets.

In real practice (e.g. as illustrated in FIG. 1), NGPs are typicallyobtained by intercalating natural graphite particles 100 with a strongacid and/or oxidizing agent to obtain a graphite intercalation compound102 (GIC) or graphite oxide (GO). The presence of chemical species orfunctional groups in the interstitial spaces between graphene planesserves to increase the inter-graphene spacing (d₀₀₂, as determined byX-ray diffraction), thereby significantly reducing the van der Waalsforces that otherwise hold graphene planes together along the c-axisdirection. The GIC or GO is most often produced by immersing naturalgraphite powder in a mixture of sulfuric acid, nitric acid (an oxidizingagent), and another oxidizing agent (e.g. potassium permanganate orsodium perchlorate). The resulting GIC (102) is actually some type ofgraphite oxide (GO) particles. This GIC or GO is then repeatedly washedand rinsed in water to remove excess acids, resulting in a graphiteoxide suspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. There are twoprocessing routes to follow after this rinsing step:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (106) that typically have athickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition). These expanded graphite flakes maybe made into a paper-like graphite mat (110).

Exfoliated graphite worms, expanded graphite flakes, and therecompressed mass of graphite worms (commonly referred to as flexiblegraphite sheet or flexible graphite foil) are all 3-D graphiticmaterials that are fundamentally different and patently distinct fromeither the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbonmaterial (graphene sheets or platelets, NGPs). Flexible graphite (FG)foils can be used as a heat spreader material, but exhibiting a maximumin-plane thermal conductivity of typically less than 500 W/mK (moretypically <300 W/mK) and in-plane electrical conductivity no greaterthan 1,500 S/cm. These low conductivity values are a direct result ofthe many defects, wrinkled or folded graphite flakes, interruptions orgaps between graphite flakes, and non-parallel flakes (e.g. SEM image inFIG. 2). Many flakes are inclined with respect to one another at a verylarge angle (e.g. mis-orientation of 20-40 degrees).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm, but more typically less than20 nm. Graphene sheets or platelets may then be made into a graphenepaper or membrane (114).

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation bas been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% byweight.

For the purpose of defining the claims of the instant application, NGPsinclude discrete sheets/platelets of single-layer and multi-layerpristine graphene, graphene oxide, or reduced graphene oxide (RGO).Pristine graphene has essentially 0% oxygen. RGO typically has an oxygencontent of 0.001%-5% by weight. Graphene oxide (including RGO) can have0.001%-50% by weight of oxygen.

It may be noted that flexible graphite foils (obtained by compressing orroll-pressing exfoliated graphite worms) for electronic device thermalmanagement applications (e.g. as a heat sink material) have thefollowing major deficiencies: (1) As indicated earlier, flexiblegraphite (FG) foils exhibit a relatively low thermal conductivity,typically <500 W/mK and more typically <300 W/mK. By impregnating theexfoliated graphite with a resin, the resulting composite exhibits aneven lower thermal conductivity (typically «200 W/mK, more typically<100 W/mK). (2) Flexible graphite foils, without a resin impregnatedtherein or coated thereon, are of low strength, low rigidity, and poorstructural integrity. The high tendency for flexible graphite foils toget torn apart makes them difficult to handle in the process of making aheat sink. As a matter of fact, the flexible graphite sheets (typically50-200 μm thick) are so “flexible” that they are not sufficiently rigidto make a fin component material for a finned heat sink. (3) Anothervery subtle, largely ignored or overlooked, but critically importantfeature of FG foils is their high tendency to get flaky with graphiteflakes easily coming off from FG sheet surfaces and emitting out toother parts of a microelectronic device. These highly electricallyconducting flakes (typically 1-200 μm in lateral dimensions and >100 nmin thickness) can cause internal shorting and failure of electronicdevices.

Similarly, solid NGPs (including discrete sheets/platelets of pristinegraphene, GO, and RGO), when packed into a film, membrane, or papersheet (114) of non-woven aggregates using a paper-making process,typically do not exhibit a high thermal conductivity unless thesesheets/platelets are closely packed and the film/membrane/paper isultra-thin (e.g. <1 μm, which is mechanically weak). This is reported inour earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007).However, ultra-thin film or paper sheets (<10 μm) are difficult toproduce in mass quantities, and difficult to handle when one tries toincorporate these thin films as a heat sink material. In general, apaper-like structure or mat made from platelets of graphene, GO, or RGO(e.g. those paper sheets prepared by vacuum-assisted filtration process)exhibit many defects, wrinkled or folded graphene sheets, interruptionsor gaps between platelets, and non-parallel platelets (e.g. SEM image inFIG. 3(B)), leading to relatively poor thermal conductivity, lowelectric conductivity, and low structural strength. These papers oraggregates of discrete NGP, GO or RGO platelets alone (without a resinbinder) also have a tendency to get flaky, emitting conductive particlesinto air.

Another prior art graphitic material is the pyrolytic graphite film,typically thinner than 100 μm. The process begins with carbonizing apolymer film (e.g. polyimide) at a carbonization temperature of400-1,500° C. under a typical pressure of 10-15 Kg/cm² for 10-36 hoursto obtain a carbonized material, which is followed by a graphitizationtreatment at 2,500-3,200° C. under an ultrahigh pressure of 100-300Kg/cm² for 1-24 hours to form a graphitic film. It is technically utmostchallenging to maintain such an ultrahigh pressure at such an ultrahightemperature. This is a difficult, slow, tedious, energy-intensive, andextremely expensive process. Furthermore, it has been difficult toproduce pyrolytic graphite film thinner than 10 μm or thicker than 100μm from a polymer such as polyimide. This thickness-related problem isinherent to this class of materials due to their difficulty in forminginto an ultra-thin (<10 μm) and thick film (>100 μm) while stillmaintaining an acceptable degree of polymer chain orientation andmechanical strength that are required of proper carbonization andgraphitization.

A second type of pyrolytic graphite is produced by high temperaturedecomposition of hydrocarbon gases in vacuum followed by deposition ofthe carbon atoms to a substrate surface. This vapor phase condensationof cracked hydrocarbons is essentially a chemical vapor deposition (CVD)process. In particular, highly oriented pyrolytic graphite (HOPG) is thematerial produced by subjecting the CVD-deposited pyro-carbon to auniaxial pressure at very high temperatures (typically 3,000-3,300° C.).This entails a thermo-mechanical treatment of combined and concurrentmechanical compression and ultra-high temperature for an extended periodof time in a protective atmosphere; a very expensive, energy-intensive,time-consuming, and technically challenging process. The processrequires ultra-high temperature equipment (with high vacuum, highpressure, or high compression provision) that is not only very expensiveto make but also very expensive and difficult to maintain. Even withsuch extreme processing conditions, the resulting HOPG still possessesmany defects, grain boundaries, and mis-orientations (neighboringgraphene planes not parallel to each other), resulting inless-than-satisfactory in-plane properties. Typically, the best preparedHOPG sheet or block typically contains many poorly aligned grains orcrystals and a vast amount of grain boundaries and defects.

Similarly, the most recently reported graphene thin film (<2 nm)prepared by catalytic CVD of hydrocarbon gas (e.g. C₂H₄) on Ni or Cusurface is not a single-grain crystal, but a poly-crystalline structurewith many grain boundaries and defects. With Ni or Cu being thecatalyst, carbon atoms obtained via decomposition of hydrocarbon gasmolecules at 800-1,000° C. are deposited onto Ni or Cu foil surface toform a sheet of single-layer or few-layer graphene that ispoly-crystalline. The grains are typically much smaller than 100 μm insize and, more typically, smaller than 10 μm in size. These graphenethin films, being optically transparent and electrically conducting, areintended for applications such as the touch screen (to replaceindium-tin oxide or ITO glass) or semiconductor (to replace silicon,Si). Furthermore, the Ni- or Cu-catalyzed CVD process does not lenditself to the deposition of more than 5 graphene planes (typically <2nm) beyond which the underlying Ni or Cu catalyst can no longer provideany catalytic effect. There has been no experimental evidence toindicate that CVD graphene layer thicker than 5 nm is possible. Both CVDgraphene film and HOPG are extremely expensive.

The above discussion clearly indicates that every prior art method orprocess for producing graphene and graphitic thin film has majordeficiencies. Hence, an urgent need exists to have a new class of carbonnano materials that are comparable or superior to graphene in terms ofthe properties essential to a current collector (e.g. electricalconductivity, thermal conductivity, contact resistance with the metalfoil), strength, and compatibility with electrolyte of an intendedbattery or supercapacitor. One must also be able to produce thesematerials more cost-effectively, faster, more scalable, and in a moreenvironmentally benign manner. The production process for such a newcarbon nano material must require a reduced amount of undesirablechemical (or elimination of these chemicals all together), shortenedprocess time, less energy consumption, reduced or eliminated effluentsof undesirable chemical species into the drainage (e.g., sulfuric acid)or into the air (e.g., SO₂ and NO₂).

Humic acid (HA) is an organic matter commonly found in soil and can beextracted from the soil using a base (e.g. KOH). HA can also beextracted from a type of coal called leonardite, which is a highlyoxidized version of lignite coal. HA extracted from leonardite containsa number of oxygenated groups (e.g. carboxyl groups) located around theedges of the graphene-like molecular center (SP² core of hexagonalcarbon structure). This material is slightly similar to graphene oxide(GO) which is produced by strong acid oxidation of natural graphite. HAhas a typical oxygen content of 5% to 42% by weight (other majorelements being carbon, hydrogen, and nitrogen). An example of themolecular structure for humic acid, having a variety of componentsincluding quinone, phenol, catechol and sugar moieties, is given inScheme 1 below (source: Stevenson F. J. “Humus Chemistry: Genesis,Composition, Reactions,” John Wiley & Sons, New York 1994).

Non-aqueous solvents for humic acid include polyethylene glycol,ethylene glycol, propylene glycol, an alcohol, a sugar alcohol, apolyglycerol, a glycol ether, an amine based solvent, an amide basedsolvent, an alkylene carbonate, an organic acid, or an inorganic acid.

The present invention also provides a process for producing a highlyoriented humic acid film (with or without externally added graphenesheets) and humic acid-derived graphitic film with a thickness from 2 nmto 30 μm (more typically and preferably from 5 nm to 10 even moretypically from 10 nm to 2 μm) and a physical density no less than 1.3g/cm³ (up to 2.2 g/cm³). This film is chemically bonded to metal foilsurfaces. In certain embodiments, the process comprises:

-   (a) preparing a dispersion of humic acid (HA) or chemically    functionalized humic acid (CHA) having HA or CHA sheets dispersed in    a liquid medium, wherein the HA sheets contain an oxygen content    higher than 5% by weight or the CHA sheets contain non-carbon    element content higher than 5% by weight; (In certain preferred    embodiments, the HA or CHA dispersion further contains graphene    sheets or molecules dispersed therein and the HA-to-graphene or    CHA-to-graphene ratio is from 1/100 to 100/1. These graphene sheets    may be selected from pristine graphene, graphene oxide, reduced    graphene oxide, graphene fluoride, graphene bromide, graphene    iodide, boron-doped graphene, nitrogen-doped graphene, chemically    functionalized graphene, or a combination thereof.)-   (b) dispensing and depositing the HA or CHA dispersion onto at least    one primary surface of a metal foil (e.g. a Cu foil) to form a wet    layer of HA or CHA, wherein the dispensing and depositing procedure    includes subjecting the dispersion to an orientation-inducing    stress; (This orientation-controlling stress, typically including a    shear stress, enables the HA/CHA sheets (or sheet-like molecules)    and graphene sheets (if present) to get aligned along planar    directions of the metal foil substrate surface (e.g. Cu foil).    Proper alignment of the HA/CHA and graphene sheets is essential to    the chemical linking or merging between two or multiple HA/CHA    sheets, or between HA/CHA sheets and graphene sheets during    subsequent heat treatments.)-   (c) partially or completely removing the liquid medium from the wet    layer of HA or CHA to form a dried HA or CHA layer having hexagonal    carbon planes and an inter-planar spacing d₀₀₂ of 0.4 nm to 1.3 nm    as determined by X-ray diffraction; and-   (d) thermally treating the dried HA or CHA layer at a first heat    treatment temperature higher than 80° C. for a sufficient period of    time to produce the highly oriented humic acid film containing    inter-connected or merged HA or CHA sheets that are substantially    parallel to one another. The humic acid film is also chemically    bonded to the metal foil surface. This is schematically illustrated    in FIG. 3(C). These HA/CHA sheets typically also have been thermally    reduced. This highly oriented humic acid film of reduced HA or CHA    may be subjected to an additional step of compressing against the    metal foil.

The process (with or without the step of compressing) can comprise anadditional step (e) of further heat-treating the humic acid film ofmerged and reduced HA or CHA at a second heat treatment temperaturehigher than the first heat treatment temperature for a sufficient periodof time to produce a graphitic film having an inter-planar spacing d₀₀₂less than 0.4 nm and an oxygen content or non-carbon element contentless than 5% by weight; and (f) compressing the graphitic film (e.g.against the Cu foil) to produce a highly conducting graphitic filmbonded to the metal foil.

In an embodiment, step (e) includes heat-treating the highly orientedhumic acid film at a second heat treatment temperature higher than thefirst heat treatment temperature (typically >300° C.) for a length oftime sufficient for decreasing an inter-plane spacing d₀₀₂ to a value offrom 0.3354 nm to 0.36 nm and decreasing the oxygen content ornon-carbon content to less than 0.5% by weight. In a preferredembodiment, the second (or final) heat treatment temperature includes atleast a temperature selected from (A) 100-300° C., (B) 300-1,500° C.,(C) 1,500-2,500° C., and/or (D) 2,500-3,200° C. Preferably, the secondheat treatment temperature includes a temperature in the range of300-1,500° C. for at least 1 hour and then a temperature in the range of1,500-3,200° C. for at least another hour.

Typically, if both the first and second heat treatment temperatures arebelow 1,500° C., the highly oriented humic acid (HOHA) film stillcontains planar molecules that are characteristic of humic acidmolecules. The highly oriented humic acid (HOHA) film containschemically bonded and merged hexagonal carbon planes, which are HA/CHAor combined HA/CHA-graphene planes. These planes (hexagonal structuredcarbon atoms having a small amount of oxygen-containing group) areparallel to one another.

This HOHA film, if exposed to a heat treatment temperature (HTT) of1,500° C. or higher for a sufficient length of time, typically no longercontains any significant amount of humic acid molecules and essentiallyall HA/CHA sheets/molecules have been converted to graphene- or grapheneoxide-like hexagonal carbon planes that are parallel to one another. Thelateral dimensions (length or width) of these planes are huge, typicallyseveral times or even orders of magnitude larger than the maximumdimensions (length/width) of the starting HA/CHA sheets. The presentlyinvented HOHA is essentially a “giant hexagonal carbon crystal” or“giant planar graphene-like layer” having all constituent graphene-likeplanes being essentially parallel to one another. This is a unique andnew class of material that has not been previously discovered,developed, or suggested to possibly exist.

The oriented HA/CHA layer (HOHA film with no HTT>1,500° C.) is itself avery unique and novel class of material that surprisingly has greatcohesion power (self-bonding, self-polymerizing, and self-crosslinkingcapability). These characteristics have not been previously taught orhinted in the prior art.

The above paragraphs have been written to describe a type of currentcollectors obtained by heat treating the HA- or HA/graphenemixture-bonded metal film as a two-layer or three-layer laminate. The HAor HA/graphene layer was not peeled off from the metal foil andheat-treated alone (without the metal foil). The resulting currentcollector does not contain a binder resin or adhesive. This type isherein referred to as Type-A current collector. This type of currentcollector can be heat-treated up to a maximum temperature close to themelting point of the underlying metal foil. However, certain metal foil(e.g. Cu, Ti, and steel) appears to be capable of catalyzing thechemical linking between HA sheets or between HA and graphene, enablingthe formation of larger HA/graphene domains and fewer defects andleading to higher thermal and electrical conductivity and structuralintegrity that otherwise could not be achieved without invoking a muchhigher heat treatment temperature.

The preparation of Type-B current collector is described in thefollowing two paragraphs:

Alternatively, the above procedures, from (a) to (d) or (e), can beconducted by depositing the dispersion of HA or HA/graphene mixture ontoa plastic film or glass surface and, upon liquid removal, the resultingdried film is peeled off from the plastic film or glass so that the filmcan be subsequently heat treated at any desired temperature. The highlyoriented HA film (after a heat treatment at a temperature from 80 to1,500° C.) or the derived graphitic film (after a heat treatment at atemperature from 1,500 to 3,200° C.), as a free-standing film, is thenbonded to one or both primary surfaces of a metal foil (e.g. Cu or Alfoil) using a binder resin or adhesive. In comparison with Type-Acurrent collector (wherein the highly oriented HA film or highlyconducting graphitic film derived therefrom is prepared by directlydepositing the thin film of HA or HA/graphene to a surface of a metalfoil and chemically bonding to this surface without using a binder),such a Type-B current collector (obtained at a comparable final heattreatment temperature) has a lower in-plane thermal conductivity, lowerin-plane electrical conductivity, higher contact resistance betweenlayers, and less durable (easier to get delaminated) in a real liquidelectrolyte environment inside a battery or supercapacitor.

In order to partially alleviating these issues, we chose to use bindermaterials that are more conducting than the typical binder resins (e.g.PVDF, SBR, etc. commonly used in lithium battery and supercapacitorindustries). These include intrinsically conductive polymers (e.g.polyaniline, polypyrrole, polythiophene, etc.), pitch (e.g. isotropicpitch, meso-phase pitch, etc.), amorphous carbon (e.g. via chemicalvapor infiltration), or a carbonized resin (heat-treating the currentcollector after the free-standing graphitic layer is bonded to the metalfoil, converting resing binder to carbon binder in situ).

The following description is for both Type-A and Type-B currentcollectors.

Step (a) entails dispersing HA/CHA sheets or molecules in a liquidmedium, which can be water or a mixture of water and an alcohol, forcertain HA or CHA molecules that contain a significant amount of —OHand/or —COOH groups at the edges and/or on the planes of the HA/CHAsheets (e.g. having an oxygen content between 20% and 47% by weight,preferably between 30% and 47%).

When the volume fraction or weight fraction of HA/CHA exceeds athreshold value, the resulting dispersion is found to contain a liquidcrystalline phase. Preferably, the HA/CHA suspension (dispersion)contains an initial volume fraction of HA/CHA sheets that exceeds acritical or threshold volume fraction for the formation of a liquidcrystal phase prior to step (b). We have observed that such a criticalvolume fraction is typically equivalent to a HA/CHA weight fraction inthe range of from 0.2% to 5.0% by weight of HA/CHA sheets in thedispersion. However, such a range of low HA/CHA contents is notparticularly amenable to the formation of the desired thin films using ascalable process, such as casting and coating. The ability to producethin films via casting or coating is highly advantageous and desirablesince large-scaled and/or automated casting or coating systems arereadily available, and the processes are known to be reliable forproduction of polymer thin films with consistently high quality.Therefore, we proceeded to conduct an in-depth and extensive study onthe suitability for casting or coating from the dispersion containing aHA/CHA-based liquid crystalline phase. We discovered that byconcentrating the dispersion to increase the HA/CHA contents from therange of 0.2% to 5.0% by weight to the range of 4% to 16% by weight ofHA/CHA sheets, we obtain a dispersion that is highly suitable tolarge-scale production of thin graphene films. Most significantly andquite unexpectedly, the liquid crystalline phase is not only preserved,but often enhanced, making it more feasible for HA/CHA sheets to beoriented along preferred orientations during the casting or coatingprocedures. In particular, the HA/CHA sheets in a liquid crystal statecontaining 4% to 16% by weight of HA/CHA sheets have the highesttendency to get readily oriented under the influence of a shear stresscreated by a commonly used casting or coating process.

Thus, in step (b), the HA/CHA suspension is formed into a thin-filmlayer preferably under the influence of a shear stress that promotes alaminar flow. One example of such a shearing procedure is casting orcoating a thin film of HA/CHA suspension using a slot-die coatingmachine. This procedure is similar to a layer of polymer solution beingcoated onto a solid substrate. The roller, “doctor's blade”, or wipercreates a shear stress when the film is shaped, or when there is arelative motion between the roller/blade/wiper and the supportingsubstrate at a sufficiently high relative motion speed. Quiteunexpectedly and significantly, such a shearing action enables theplanar HA/CHA sheets to well align along, for instance, a shearingdirection. Further surprisingly, such a molecular alignment state orpreferred orientation is not disrupted when the liquid components in theHA/CHA suspension are subsequently removed to form a well-packed layerof highly aligned HA/CHA sheets that are at least partially dried. Thedried layer has a high birefringence coefficient between an in-planedirection and the normal-to-plane direction.

The present invention includes the discovery of a facile amphiphilicself-assembly approach to fabricate HA/CHA-based thin films with desiredhexagonal plane orientation. HA containing 5-46% by weight of oxygen maybe considered a negatively charged amphiphilic molecule due to itscombination of hydrophilic oxygen-containing functional groups and ahydrophobic basal plane. For a CHA, the functional groups can be made tobe hydrophilic or hydrophobic. The successful preparation of the HA/CHAfilms with unique hexagonal, graphene-like plane orientations does notrequire complex procedures. Rather, it is achieved by tailoring HA/CHAsynthesis and manipulating the liquid crystalline phase formation anddeformation behaviors to enable the self-assembly of HA/CHA sheets in aliquid crystalline phase.

The HA/CHA suspension was characterized using atomic force microscopy(AFM), Raman spectroscopy, and FTIR to confirm its chemical state.Finally, the presence of lyotropic meso-morphism of HA sheets (liquidcrystalline HA phase) in aqueous solution was demonstrated throughcross-polarized light observation.

Two major aspects are considered to determine if a 1-D or 2-D speciescan form a liquid crystalline phase in a liquid medium: the aspect ratio(the length/width/diameter-to-thickness ratio) and sufficientdispersibility or solubility of this material in the liquid medium. HAor CHA sheets feature high anisotropy, with monatomic or few-atomthickness (t) and normally micrometer-scale lateral width (w). Accordingto Onsager's theory, high aspect ratio 2D sheets can form liquidcrystals in dispersions, when their volume fraction exceeds a criticalvalue:V _(c)≈4t/w  (Eq. 1)Given the thickness of a graphene-like plane being 0.34 nm and a widthof 1 μm, the required critical volume would beV_(c)≈4t/w=4×0.34/1,000=1.36×10⁻³=0.136%. However, pristine graphenesheets are not soluble in water and poorly dispersible in common organicsolvents (maximum volume fraction, V_(m), ˜0.7×10⁻⁵ inN-methylpyrrolidone (NMP) and ˜1.5×10⁻⁵ in ortho-dichlorobenzene), owingto their strong π-π stacking attraction. Fortunately, the molecularstructure of HA or CHA can be made to exhibit good dispersibility inwater and polar organic solvents, such as alcohol, N,N-dimethylformamide (DMF) and NMP, due to the numerous oxygen-containingfunctional groups attached to its edges. Naturally occurring HA (e.g.that from coal) is also highly soluble in non-aqueous solvents for humicacid include polyethylene glycol, ethylene glycol, propylene glycol, analcohol, a sugar alcohol, a polyglycerol, a glycol ether, an amine basedsolvent, an amide based solvent, an alkylene carbonate, an organic acid,an inorganic acid, or a mixture thereof.

Although, presumably the critical volume fraction of HA/CHA can be lowerthan 0.2% or critical weight fraction lower than 0.3% according totheoretical prediction, we have observed that the critical weightfractions for HA/CHA sheets to form liquid crystals are significantlyhigher than 0.4% by weight. The most stable liquid crystals are presentwhen the weight fraction of HA/CHA sheets is in the range of 0.6%-5.0%,which enable high stability over a wide temperature range. To study theeffect of HA/CHA size on the formation of its liquid crystallinestructure, HA/CHA samples were prepared using a pH-assisted selectivesedimentation technique. The lateral sizes of HA/CHA sheets wereassessed by dynamic light scattering (DLS) via three differentmeasurement modes, as well as AFM.

During the investigation of HA/CHA liquid crystals we made an unexpectedbut highly significant discovery: The liquid crystalline phase of HA/CHAsheets in water and other solvents can be easily disrupted or destroyedwith mechanical disturbances (e.g. mechanical mixing, shearing,turbulence flow, etc.). The mechanical stability of these liquidcrystals can be significantly improved if the concentration of HA/CHAsheets is gradually increased to above 5% (preferably from 5% to 16% byweight) by carefully removing (e.g. vaporizing) the liquid mediumwithout mechanically disturbing the liquid crystalline structure. Wefurther observed that with a HA/CHA weight fraction in this range of5-16%, HA/CHA sheets are particularly amenable to forming desiredorientations during casting or coating to form thin films.

Thermodynamically, the process of amphiphilic HA/CHA self-assembly intoa liquid crystalline phase is an interplay of the enthalpy change (ΔH)and entropy change (ΔS) as shown in Eq. (2):ΔG _(self-assembly) −ΔH _(self-assembly) −TΔS _(self-assembly)  (2)Previous studies into the thermodynamic driving force for amphiphilicself-assembly into liquid crystal phases indicate that the entropiccontribution plays a dominant role, while the enthalpy change isunfavorable in most cases. Onsager's theory predicts that high aspectratio particles can form liquid crystal phases above a critical volumefraction due to a net gain in entropy as the loss of orientationalentropy is compensated for by an increased translational entropy.Specifically, higher aspect ratio particles favor the formation oflong-range liquid crystalline phases. Another possible reason for theHA/CHA aspect ratio effect could be the structural corrugation of HA/CHAsheets in solvent as the restoring force originated from bending thesheets is much weaker than that along the sheet. It was found that thedegree of HA/CHA corrugated morphology in solvent could be furtherenhanced if its aspect ratio is increased. This corrugated configurationwill significantly affect both the intra and intermolecular interactionsof HA/CHA in suspension.

To achieve long-range ordering in an aqueous dispersion, well-exfoliatedHA/CHA sheets with strong long-range electrostatic repulsion arerequired. Formation of liquid crystal structures out of colloidalparticles typically requires a delicate balance of long-range repulsiveforces, such as electrostatic forces, and short-range attractive forces,such as van der Waals forces and π-π interactions. If the long-rangerepulsive forces are not strong enough to overcome the short-rangeattractive forces, aggregation of colloidal particles or only weakformation of a lyotropic liquid crystal with small periodicity willinevitably occur. In the HA/CHA aqueous dispersion, long-range repulsiveinteractions are offered by the electrical double layers formed by theionized oxygen functional groups. Although HA/CHA sheets still contain aconsiderable portion of hydrophobic domains, attractive π-π interactionsand van der Waals forces can be effectively overcome by adjusting thelong-range electrostatic repulsive forces

The chemical composition of HA/CHA plays an important role in tailoringthe electrostatic interaction in an aqueous or organic solventdispersion. The increase of surface charge density will lead to anincrease in the strength of the electrostatic repulsion against theattractive forces. The ratio of the aromatic and oxygenated domains canbe easily tuned by the level of hexagonal carbon plane oxidation orchemical modification. The Fourier transform infrared spectroscopy underattenuated total reflectance mode (FTIR-ATR) results of the HA/CHAindicate that oxidized species (hydroxyl, epoxy, and carboxyl groups)exist on the HA/CHA surfaces. Thermogravimetric analysis (TGA) innitrogen was used to probe the oxygen functional group density on theHA/CHA surface. For a highly oxidized HA, a mass loss of ˜28% by weightis found at around 250° C. and is attributed to the decomposition oflabile oxygen-containing species. Below 160° C., a mass loss of ˜16 wt %is observed, corresponding to desorption of physically absorbed water.The X-ray photoelectron spectroscopy (XPS) result of HA shows that anatomic ratio of C/O is about 1.9. This suggests that the HA has arelatively high density of oxygen functional groups. In addition, wealso prepared HA containing a lower density of oxygen functional groupsby simply varying the thermal or chemical reduction time and temperatureof heavily oxidized HA (e.g. from leonardite coal). We have observedthat liquid crystals can be found with oxygen weight fractionspreferentially in the range of 5%-40%, more preferably 5%-30%, and mostpreferably 5%-20%.

The colloidal interaction between HA sheets can be significantlyinfluenced by the ionic strength, because the Debye screening length(κ−1) can be effectively increased by reducing the concentration of freeions surrounding HA sheets. The electrostatic repulsion of the HA liquidcrystal in water could decrease as the salt concentration increases. Asa result, more water is expelled from the HA interlamellar space with anaccompanying reduction in d spacing. Thus, ionic impurities in the HAdispersions should be sufficiently removed, as it is a crucial factorinfluencing the formation of HA liquid crystal structure.

However, we have also found that introduction of some small amount ofpolymer (up to 10% by weight, but preferably up to 5% by weight, andmost preferably up to only 2%) can help stabilize the liquid crystalphase when the HA/CHA dispersion is subjected to casting or coatingoperations. With proper functional groups and concentrations, the GO/CFGorientation in the resultant film could be enhanced. This also has neverbeen taught or hinted in previous open or patent literature.

The dried HA/CHA layer may then be subjected to heat treatments. Aproperly programmed heat treatment procedure can involve at least twoheat treatment temperatures (first temperature for a period of time andthen raised to a second temperature and maintained at this secondtemperature for another period of time), or any other combination of atleast two heat treatment temperatures (HTT) that involve an initialtreatment temperature (first temperature) and a final HTT, higher thanthe first.

The first heat treatment temperature is for chemical linking and thermalreduction of HA/CHA and is conducted at the first temperature of >80° C.(can be up to 1,000° C., but preferably up to 700° C., and mostpreferably up to 300° C.). This is herein referred to as Regime 1:

-   Regime 1 (up to 300° C.): In this temperature range (the initial    chemical linking and thermal reduction regime), chemical    combination, polymerization (edge-to-edge merging), and    cross-linking between adjacent HA/CHA sheets begin to occur.    Multiple HA/CHA sheets are packed and chemically bonded together    side by side and edge to edge to form an integrated layer of    graphene oxide-like entity. In addition, a HA/CHA layer primarily    undergoes thermally-induced reduction reactions, leading to a    reduction of oxygen content to approximately 5% or lower. This    treatment results in a reduction of inter-graphene spacing from    approximately 0.8-1.2 nm (as dried) down to approximately 0.4 nm,    and an increase in in-plane thermal conductivity from approximately    100 W/mK to 500 W/mK. Even with such a low temperature range, some    chemical linking between HA/CHA sheets occurs. The HA/CHA sheets    remain well-aligned, but the inter-graphene plane spacing remains    relatively large (0.4 nm or larger). Many 0-containing functional    groups survive.-   The highest or final HTT that the GO mass experiences may be divided    into three distinct HTT regimes:-   Regime 2 (300° C.-1,500° C.): In this mainly chemical linking    regime, additional thermal reduction and extensive chemical    combination, polymerization, and cross-linking between adjacent    HA/CHA sheets occur. The chemical linking between HA/CHA and    graphene sheets (e.g. GO sheets), if present, also occurs. The    oxygen content is reduced to typically below 1% after chemical    linking, resulting in a reduction of inter-graphene spacing to    approximately 0.35 nm. This implies that some initial graphitization    has already begun at such a low temperature, in stark contrast to    conventional graphitizable materials (such as carbonized polyimide    film) that typically require a temperature as high as 2,500° C. to    initiate graphitization. This is another distinct feature of the    presently invented HOHA film and its production processes. These    chemical linking reactions result in an increase in in-plane thermal    conductivity to 850-1,250 W/mK, and/or in-plane electrical    conductivity to 3,500-4,500 S/cm.-   Regime 3 (1,500-2,500° C.): In this ordering and re-graphitization    regime, extensive graphitization or graphene plane merging occurs,    leading to significantly improved degree of structural ordering. As    a result, the oxygen content is reduced to typically 0.01% and the    inter-graphene spacing to approximately 0.337 nm (achieving degree    of graphitization from 1% to approximately 80%, depending upon the    actual HTT and length of time). The improved degree of ordering is    also reflected by an increase in in-plane thermal conductivity    to >1,300-1,500 W/mK, and/or in-plane electrical conductivity to    5,000-7,000 S/cm. Regime 4 (higher than 2,500° C.): In this    re-crystallization and perfection regime, extensive movement and    elimination of grain boundaries and other defects occur, resulting    in the formation of nearly perfect single crystals or    poly-crystalline graphene crystals with huge grains, which can be    orders of magnitude larger than the original grain sizes of the    starting HA/CHA sheets. The oxygen content is essentially    eliminated, typically 0.01%-0.1%. The inter-graphene spacing is    reduced to down to approximately 0.3354 nm (degree of graphitization    from 80% to nearly 100%), corresponding to that of a perfect    graphite single crystal. Quite interestingly, the graphene    poly-crystal has all the graphene planes being closely packed and    bonded, and all the planes are aligned along one direction, a    perfect orientation. Such a perfectly oriented structure has not    been produced even with the HOPG that was produced by subjecting    pyrolytic graphite concurrently to an ultra-high temperature (3,400°    C.) under an ultra-high pressure (300 Kg/cm²). The highly oriented    graphene structure can achieve such a highest degree of perfection    with a significantly lower temperature and an ambient (or slightly    higher compression) pressure. The structure thus obtained exhibits    an in-plane thermal conductivity from 1,500 up to slightly >1,700    W/mK, and in-plane electrical conductivity to a range from 15,000 to    20,000 S/cm.    The presently invented highly oriented HA-derived structure can be    obtained by heat-treating the HA/CHA layer with a temperature    program that covers at least the first regime (typically requiring    1-24 hours in this temperature range), more commonly covers the    first two regimes (1-10 hours preferred), still more commonly the    first three regimes (preferably 0.5-5 hours in Regime 3), and most    commonly all the 4 regimes (Regime 4, for 0.5 to 2 hour, may be    implemented to achieve the highest conductivity).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. The HOHAhaving a d₀₀₂ higher than 0.3440 nm reflects the presence ofoxygen-containing functional groups (such as —OH, >O, and —COOH ongraphene-like plane surfaces) that act as a spacer to increase theinter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the presently invented HOHA-derived graphitic film andconventional graphite crystals is the “mosaic spread,” which isexpressed by the full width at half maximum of a rocking curve (X-raydiffraction intensity) of the (002) or (004) reflection. This degree ofordering characterizes the graphite or graphene crystal size (or grainsize), amounts of grain boundaries and other defects, and the degree ofpreferred grain orientation. A nearly perfect single crystal of graphiteis characterized by having a mosaic spread value of 0.2-0.4. Most of ourHOHA-derived graphitic samples have a mosaic spread value in this rangeof 0.2-0.4 (if produced with a heat treatment temperature (HTT) no lessthan 2,500° C.). However, some values are in the range of 0.4-0.7 if theHTT is between 1,500 and 2,500° C., and in the range of 0.7-1.0 if theHTT is between 300 and 1,500° C.

HA or graphene may be functionalized through various chemical routes. Inone preferred embodiment, the resulting functionalized HA orfunctionalized graphene (collectively denoted as Gn) may broadly havethe following formula(e):[Gn]-R_(m)wherein m is the number of different functional group types (typicallybetween 1 and 5), R is selected from SO₃H, COOH, NH₂, OH, R′CHOH, CHO,CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃, Si(—OR′—)_(y)R′₃-y,Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂ and Mg—X; wherein y is aninteger equal to or less than 3, R′ is hydrogen, alkyl, aryl,cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ isfluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, Xis halide, and Z is carboxylate or trifluoroacetate.

Assuming that a polymer, such as epoxy resin, and HA or graphene sheetscan be combined to make a coating composition, then the function group—NH₂ is of particular interest. For example, a commonly used curingagent for epoxy resin is diethylenetriamine (DETA), which can have 2 ormore —NH₂ groups. One of the —NH₂ groups may be bonded to the edge orsurface of a graphene sheet and the remaining un-reacted —NH₂ groupswill be available for reacting with epoxy resin later. Such anarrangement provides a good interfacial bonding between the HA (orgraphene) sheet and the resin additive.

Other useful chemical functional groups or reactive molecules may beselected from the group consisting of amidoamines, polyamides, aliphaticamines, modified aliphatic amines, cycloaliphatic amines, aromaticamines, anhydrides, ketimines, diethylenetriamine (DETA),triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA),polyethylene polyamine, polyamine epoxy adduct, phenolic hardener,non-brominated curing agent, non-amine curatives, and combinationsthereof. These functional groups are multi-functional, with thecapability of reacting with at least two chemical species from at leasttwo ends. Most importantly, they are capable of bonding to the edge orsurface of graphene or HA using one of their ends and, during subsequentcuring stage, are able to react with a resin at one or two other ends.

The above-described [Gn]-R_(m) may be further functionalized. Theresulting CFGs include compositions of the formula:[Gn]-A_(m),where A is selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y,—CR′1-OY, N′Y or C′Y, and Y is an appropriate functional group of aprotein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide,an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitoror the transition state analog of an enzyme substrate or is selectedfrom R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃,R′Si(—OR′—)_(y)R′_(3-y), R′Si(O—SiR′₂—)OR′, R′—R″, R′—N—CO,(C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, R′, andw is an integer greater than one and less than 200.

The HA and/or graphene sheets may also be functionalized to producecompositions having the formula:[Gn]-[R′-A]_(m)where m, R′ and A are as defined above. The compositions of theinvention also include CHAs upon which certain cyclic compounds areadsorbed. These include compositions of matter of the formula:[Gn]-[X—R_(a)]_(m)where a is zero or a number less than 10, X is a polynuclear aromatic,polyheteronuclear aromatic or metallopolyheteronuclear aromatic moietyand R is as defined above. Preferred cyclic compounds are planar. Morepreferred cyclic compounds for adsorption are porphyrins andphthalocyanines. The adsorbed cyclic compounds may be functionalized.Such compositions include compounds of the formula:[Gn]-[X-A_(a)]_(m)where m, a, X and A are as defined above.

The functionalized HA or graphene of the instant invention can bedirectly prepared by sulfonation, electrophilic addition to deoxygenatedGO surfaces, or metallation. The graphene or HA sheets can be processedprior to being contacted with a functionalizing agent. Such processingmay include dispersing the graphene or HA sheets in a solvent. In someinstances the sheets may then be filtered and dried prior to contact.One particularly useful type of functional groups is the carboxylic acidmoieties, which naturally exist on the surfaces of HAs if they areprepared from acid intercalation route discussed earlier. If anadditional amount of carboxylic acid is needed, the HA sheets may besubjected to chlorate, nitric acid, or ammonium persulfate oxidation.

Carboxylic acid functionalized graphene sheets are particularly usefulbecause they can serve as the starting point for preparing other typesof functionalized graphene or HA sheets. For example, alcohols or amidescan be easily linked to the acid to give stable esters or amides. If thealcohol or amine is part of a di- or poly-functional molecule, thenlinkage through the O— or NH— leaves the other functionalities aspendant groups. These reactions can be carried out using any of themethods developed for esterifying or aminating carboxylic acids withalcohols or amines as known in the art. Examples of these methods can befound in G. W. Anderson, et al., J. Amer. Chem. Soc. Vol. 86, pp.1839-1842 (1964), which is hereby incorporated by reference in itsentirety. Amino groups can be introduced directly onto graphitic fibrilsby treating the fibrils with nitric acid and sulfuric acid to obtainnitrated fibrils, then chemically reducing the nitrated form with areducing agent, such as sodium dithionite, to obtainamino-functionalized fibrils.

We have found that the aforementioned functional groups can be attachedto HA or graphene sheet surfaces or edges for one or several of thefollowing purposes: (a) for improved dispersion of graphene or HA in adesired liquid medium; (b) enhanced solubility of graphene or HA in aliquid medium so that a sufficient amount of graphene or HA sheets canbe dispersed in this liquid that exceed the critical volume fraction forliquid crystalline phase formation; (c) enhanced film-forming capabilityso that thin film of otherwise discrete sheets of graphene or HA can becoated or cast; (d) improved capability of graphene or HA sheets to getoriented due to modifications to the flow behaviors; and (e) enhancedcapability for graphene or HA sheets to get chemically linked and mergedinto larger or wider graphene planes.

The present invention also provides a rechargeable battery that containsa presently invented graphene oxide thin film-bonded metal foil as ananode current collector and/or a cathode current collector. This can beany rechargeable battery, such as a zinc-air cell, a nickel metalhydride cell, a sodium-ion cell, a sodium metal cell, a magnesium-ioncell, or a magnesium metal cell, just to name a few. This inventedbattery can be a rechargeable lithium battery containing the unitarygraphene layer as an anode current collector or a cathode currentcollector, which lithium battery can be a lithium-sulfur cell, alithium-selenium cell, a lithium sulfur/selenium cell, a lithium-ioncell, a lithium-air cell, a lithium-graphene cell, or a lithium-carboncell. Another embodiment of the invention is a capacitor containing thecurrent collector of the present invention as an anode current collectoror a cathode current collector, which capacitor is a symmetricultracapacitor, an asymmetric ultracapacitor cell, a hybridsupercapacitor-battery cell, or a lithium-ion capacitor cell

As an example, the present invention provides a rechargeablelithium-metal cell composed of a current collector at the anode, alithium film or foil as the anode, a porous separator/electrolyte layer,a cathode containing a cathode active material (e.g. lithium-free V₂O₅and MnO₂), and a current collector. Either or both the anode currentcollector and cathode current collector can be a HA-based currentcollector of the present invention (i.e. derived from highly orientedthin film of HA or a HA/graphene mixture).

Another example of the present invention is a lithium-ion capacitor (orhybrid supercapacitor) composed of a current collector at the anode, agraphite or lithium titanate anode, a porous separator soaked withliquid or gel electrolyte, a cathode containing a cathode activematerial (e.g. activated carbon having a high specific surface area),and a current collector. Again, either or both the anode currentcollector and cathode current collector can be a HA-based currentcollector of the present invention.

Yet another example of the present invention is another lithium-ioncapacitor or hybrid supercapacitor, which is composed of a currentcollector at the anode, a graphite anode (and a sheet of lithium foil aspart of the anode), a porous separator soaked with liquid electrolyte, acathode containing a cathode active material (e.g. activated carbonhaving a high specific surface area), and a current collector. Again,either or both the anode current collector and cathode current collectorcan be a HA-based current collector of the present invention.

Still another example of the present invention is a lithium-graphenecell composed of a current collector at the anode, a porous,nano-structured anode (e.g. comprising graphene sheets having highsurface areas upon which returning lithium ions can deposit during cellrecharge, which are mixed with surface-stabilized lithium powderparticles, or having a sheet of lithium foil attached to thenano-structure), a porous separator soaked with liquid electrolyte, acathode containing a graphene-based cathode active material (e.g.graphene, graphene oxide, or graphene fluoride sheets having highspecific surface areas to capture lithium ions during cell discharge),and a cathode current collector. Again, either or both the anode currentcollector and cathode current collector can be a HA-based currentcollector of the present invention.

Example 1: Humic Acid and Reduced Humic Acid from Leonardite

Humic acid can be extracted from leonardite by dispersing leonardite ina basic aqueous solution (pH of 10) with a very high yield (in the rangeof 75%). Subsequent acidification of the solution leads to precipitationof humic acid powder. In an experiment, 3 g of leonardite was dissolvedby 300 ml of double deionized water containing 1M KOH (or NH₄OH)solution under magnetic stirring. The pH value was adjusted to 10. Thesolution was then filtered to remove any big particles or any residualimpurities.

The resulting humic acid dispersion, containing HA alone or HA with thepresence of graphene oxide sheets (GO prepared in Example 3 describedbelow), was coated onto a Cu foil or Ti foil surface form a series ofHA-bonded Cu foil or Ti foil films for subsequent heat treatments toobtain Type-A current collectors.

For comparison, similar films were cast onto glass surface and thenpeeled off prior to subsequent heat treatments for the preparation ofType-B current collectors.

Example 2: Preparation of Humic Acid from Coal and HA-Bonded Metal FoilCurrent Collectors

In a typical procedure, 300 mg of coal was suspended in concentratedsulfuric acid (60 ml) and nitric acid (20 ml), and followed by cupsonication for 2 h. The reaction was then stirred and heated in an oilbath at 100 or 120° C. for 24 h. The solution was cooled to roomtemperature and poured into a beaker containing 100 ml ice, followed bya step of adding NaOH (3M) until the pH value reached 7.

In one experiment, the neutral mixture was then filtered through a0.45-mm polytetrafluoroethylene membrane and the filtrate was dialyzedin 1,000 Da dialysis bag for 5 days. For the larger humic acid sheets,the time can be shortened to 1 to 2 h using cross-flow ultrafiltration.After purification, the solution was concentrated using rotaryevaporation to obtain solid humic acid sheets. These humic sheets aloneand their mixtures with graphene sheets were re-dispersed in a solvent(ethylene glycol and alcohol, separately) to obtain several dispersionsamples for subsequent casting or coating onto Al foil and stainlesssteel foils. Both Type-A and Type B current collectors were prepared.

Example 3: Preparation of Graphene Oxide (GO) Sheets from NaturalGraphite Powder

Natural graphite from Ashbury Carbons was used as the starting material.GO was obtained by following the well-known modified Hummers method,which involved two oxidation stages. In a typical procedure, the firstoxidation was achieved in the following conditions: 1100 mg of graphitewas placed in a 1000 mL boiling flask. Then, 20 g of K₂S₂O₈, 20 g ofP₂O₅, and 400 mL of a concentrated aqueous solution of H₂SO₄ (96%) wereadded in the flask. The mixture was heated under reflux for 6 hours andthen let without disturbing for 20 hours at room temperature. Oxidizedgraphite was filtered and rinsed with abundant distilled water until apH value >4.0 was reached. A wet cake-like material was recovered at theend of this first oxidation.

For the second oxidation process, the previously collected wet cake wasplaced in a boiling flask that contains 69 mL of a concentrated aqueoussolution of H₂SO₄ (96%). The flask was kept in an ice bath as 9 g ofKMnO₄ was slowly added. Care was taken to avoid overheating. Theresulting mixture was stirred at 35° C. for 2 hours (the sample colorturning dark green), followed by the addition of 140 mL of water. After15 min, the reaction was halted by adding 420 mL of water and 15 mL ofan aqueous solution of 30 wt % H₂O₂. The color of the sample at thisstage turned bright yellow. To remove the metallic ions, the mixture wasfiltered and rinsed with a 1:10 HCl aqueous solution. The collectedmaterial was gently centrifuged at 2700 g and rinsed with deionizedwater. The final product was a wet cake that contained 1.4 wt % of GO,as estimated from dry extracts. Subsequently, liquid dispersions of GOplatelets were obtained by lightly sonicating wet-cake materials, whichwere diluted in deionized water.

On a separate basis, water suspensions containing mixtures of GO andhumic acid at various GO proportions (1%, 10%, 20%, 30%, 40%, 50%, 60%,70%, 80%, 90%, and 99%) were prepared and slot-die coated to producethin films of various compositions.

Example 4: Preparation of Oriented Films Containing a Mixture of HumicAcid and Pristine Graphene Sheets (0% Oxygen)

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.Pristine graphene is essentially free from any non-carbon elements.

The suspension after ultrasonication contains pristine graphene sheetsdispersed in water and s surfactant dissolved therein. Humic acid wasthen added into the suspension and the resulting mixture suspension wasfurther ultrasonicated for 10 minutes to facilitate uniform dispersionand mixing. The dispersion was then coated onto Cu and Ti foil and, forcomparison, onto glass and PET films, prior to heat treatments.

Example 5: Preparation of Highly Oriented Graphitic Films from Mixturesof Graphene Fluoride Sheets and Humic Acid

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7 days a gray-beige productwith approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol and ethanol, separately)and subjected to an ultrasound treatment (280 W) for 30 min, leading tothe formation of homogeneous yellowish dispersions. Humic acid was thenadded to these dispersions at various HA-to-GF ratios. The dispersionswere then made into thin films supported by Cu foil using comma coating.The highly oriented HA films were then heat-treated to various extentsto obtain highly conducting graphitic films.

Example 6: Preparation of the HOHA Films Containing NitrogenatedGraphene Sheets and Humic Acid

Graphene oxide (GO), synthesized in Example 3, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The products wereobtained with graphene/urea mass ratios of 1:0.5, 1:1 and 1:2. and thenitrogen contents of these samples were 14.7, 18.2 and 17.5 wt. %respectively as determined by elemental analysis. These nitrogenatedgraphene sheets remain dispersible in water. Various amounts of HA,having oxygen contents of 20.5% to 45%, were added into the suspensions.

The resulting suspension of nitrogenated graphene-HA dispersions werethen coated onto a plastic film substrate to form wet films, which werethen dried and peeled off from the plastic film and subjected to heattreatments at various heat treatment temperatures, from 80 to 2,900° C.to obtain highly oriented humic acid (HOHA) films (if final HTT<1,500°C.) or highly ordered and conducting graphitic films (if 1,500° C. orhigher). These films were then bonded to Ti and Cu surfaces, using aresin binder, to make Type-B current collectors. Additionally, forcomparison purposes, some amounts of suspension of nitrogenatedgraphene-HA dispersions were also coated onto Ti and Cu foil surfaces toform wet films, which were then dried and heat-treated up to 1,500° C.and 1,250° C., respectively.

Example 7: Preparation of Nematic Liquid Crystals from Humic Acid Sheetsand Highly Conducting Films Produced Therefrom

Humic acid aqueous dispersions were prepared by dispersing HA sheets indeionized water by mild sonication. Any acidic or ionic impurities inthe dispersions were removed by dialysis, which is a crucial step forliquid-crystal formation.

A low-concentration dispersion (typically 0.05-0.6 wt. %) immobilizedfor a sufficiently long time (usually more than 2 weeks) macroscopicallyphase-separated into two phases. While the low-density top phase wasoptically isotropic, the high-density bottom phase demonstratedprominent optical birefringence between two crossed polarizers. Atypical nematic schlieren texture consisting of dark and bright brusheswas observed in the bottom phase. This is biphasic behavior, where anisotropic phase and nematic phase coexist. The compositional range forthe biphase was significantly broad because of the large polydispersityof the HA molecules. It may be noted that ionic strength and pH valuessignificantly influence the stability of HA liquid crystals. Theelectrostatic repulsion from the dissociated surface functional groupssuch as carboxylate plays a crucial role in the stability of HA liquidcrystals. Thus, reducing repulsive interaction by increasing ionicstrength or lowering pH values increased the coagulation of HA sheets.

We observed that substantially all HA sheets form a liquid crystal phasewhen HA sheets occupy a weight fraction of 1.1%, and the liquid crystalscan be preserved by gradually increasing the concentration of HA to therange of from 6% to 16%. The prepared humic acid dispersion exhibited aninhomogeneous, chocolate-milk-like appearance to the naked eye. Thismilky appearance can be mistaken for aggregation or precipitation of thegraphene oxide but, in fact, it is a nematic liquid crystal.

By dispensing and coating the HA suspension on a polyethyleneterephthalate (PET) film in a slurry coater and removing the liquidmedium from the coated film we obtained a thin film of dried HA. Thedried film was peeled off from the PET film to become a free-standingfilm prior to heat treating. Additionally, HA suspension was also coatedon Cu foil or Ti surfaces and then dried. Each film (both thefree-standing film peeled off from PET and the Ti- or Cu-supported film)was then subjected to different heat treatments, which typically includea chemical linking and thermal reduction treatment at a firsttemperature of 80° C. to 300° C. for 1-10 hours, and at a secondtemperature of 1,500° C.-2,850° C. for 0.5-5 hours. The Cu-supportedfilm and Ti-supported film were heat-treated up to only 1,250° C. and1,500° C., respectively. With these heat treatments, also under acompressive stress, the HOHA films were transformed into highlyconducting graphitic films (HOGF).

The internal structures (crystal structure and orientation) of severaldried HA layers (HOHA films), and the HOGF at different stages of heattreatments were investigated. X-ray diffraction curves of a layer ofdried HOHA prior to the heat treatment, a HOHA film thermally treated at150° C. for 5 hours, and the resultant HOGF were obtained. The peak atapproximately 2θ=12° of the dried HOHA layer corresponds to aninter-graphene spacing (d₀₀₂) of approximately 0.75 nm. With some heattreatment at 150° C., the dried film exhibits the formation of a humpcentered at 22°, indicating that it has begun the process of decreasingthe inter-planar spacing, indicating the beginning of chemical linkingand ordering processes. With a heat treatment temperature of 2,500° C.for one hour, the d₀₀₂ spacing of the films (not bonded to a metal foil)has decreased to approximately 0.336, close to 0.3354 nm of a graphitesingle crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing of the films not bonded to metal surfaces is decreased toapproximately to 0.3354 nm, identical to that of a graphite singlecrystal. In addition, a second diffraction peak with a high intensityappears at 2θ=55° corresponding to X-ray diffraction from (004) plane.The (004) peak intensity relative to the (002) intensity on the samediffraction curve, or the I(004)/I(002) ratio, is a good indication ofthe degree of crystal perfection and preferred orientation of grapheneplanes. It is well-known in the art that the (004) peak is eithernon-existing or relatively weak, with the I(004)/I(002) ratio <0.1, forall conventional graphitic materials heat treated at a temperature lowerthan 2,800° C. The I(004)/I(002) ratio for the graphitic materials heattreated at 3,000-3,250° C. (e.g., highly oriented pyrolytic graphite,HOPG) is in the range of 0.2-0.5. In contrast, a HOGF prepared from theHA liquid crystal-based film with a final HTT of 2,750° C. for one hourexhibits a I(004)/I(002) ratio of 0.77 and a Mosaic spread value of0.21, indicating a practically perfect graphene single crystal with anexceptionally high degree of preferred orientation.

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Most of our HA-derived HOGF have a mosaicspread value in this range of 0.2-0.4 when produced using a final heattreatment temperature no less than 2,500° C.

It may be noted that the I(004)/I(002) ratio for all tens of flexiblegraphite foil compacts investigated are all «0.05, practicallynon-existing in most cases. The I(004)/I(002) ratio for all graphenepaper/membrane samples prepared with a vacuum-assisted filtration methodis <0.1 even after a heat treatment at 3,000° C. for 2 hours. Theseobservations have further confirmed the notion that the presentlyinvented HOHA film is a new and distinct class of material that isfundamentally different from any pyrolytic graphite (PG), flexiblegraphite (FG), and paper/film/membrane of conventional graphene/GO/RGOsheets/platelets (NGPs).

The inter-graphene spacing values of both the HA liquid crystalsuspension-derived HOGF samples obtained by heat treating at varioustemperatures over a wide temperature range are summarized in FIG. 5(A).Corresponding oxygen content values are shown in FIG. 5(B). In order toshow the correlation between the inter-graphene spacing and the oxygencontent, the data in FIG. 5(A) and FIG. 5(B) are re-plotted in FIG.5(C). A close scrutiny of FIG. 5(A), FIG. 5(B) and FIG. 5(C) indicatethat there are four HTT ranges (100-300° C.; 300-1,500° C.; 1,500-2,000°C., and >2,000° C.) that lead to four respective oxygen content rangesand inter-graphene spacing ranges. The thermal conductivity of the HAliquid crystal-derived HOGF specimens and the corresponding sample offlexible graphite (FG) foil sheets, also plotted as a function of thesame final heat treatment temperature range, is summarized in FIG. 5(D).All these samples have comparable thickness values.

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-planar spacingto below 0.4 nm, getting closer and closer to that of natural graphiteor that of a graphite single crystal. The beauty of this approach is thenotion that this HA liquid crystal suspension strategy has enabled us tore-organize, re-orient, and chemically merge the planar HA sheets into aunified structure with all the graphene-like planes now being larger inlateral dimensions (significantly larger than the length and width ofthe hexagonal carbon planes in the original HA molecules) andessentially parallel to one another. This has given rise to a thermalconductivity already 300-400 W/mK (with a HTT of 500° C.) and >623 W/mk(from HA only) or >900 W/mk (from mixture of HA+GO) with a HTT of 700°C., which is more than 3- to 4-fold greater than the value (200 W/mK) ofthe corresponding flexible graphite foil. Furthermore, the tensilestrength of the HOGF samples can reach 90-125 MPa (FIG. 7(A)).

With a HTT as low as 1,000° C., the resulting highly oriented HA filmexhibits a thermal conductivity of 756 W/mK (from HA alone) and 1,105W/mK (from a HA-GO mixture), respectively. This is in stark contrast tothe observed 268 W/mK of the flexible graphite foil with an identicalheat treatment temperature. As a matter of fact, no matter how high theHTT is (e.g. even as high as 2,800° C.), the flexible graphite foil onlyshows a thermal conductivity lower than 600 W/mK. At a HTT of 2,800° C.,the presently invented HOGF layer delivers a thermal conductivity of1,745 W/mK for a layer derived from a mixture of HA and GO (FIG. 4(A)and FIG. 5(D)). It may be further noted that, as indicated in FIG. 4(A),the thermal conductivity values of HA/GO mixture-derived graphitic filmsare consistently higher than those of corresponding graphitic filmsderived from graphene oxide. This surprising effect is further discussedin Example 8.

Scanning electron microscopy (SEM), transmission electron microscopy(TEM) pictures of lattice imaging of the graphene layer, as well asselected-area electron diffraction (SAD), bright field (BF), anddark-field (DF) images were also conducted to characterize the structureof unitary graphene materials. For measurement of cross-sectional viewsof the film, the sample was buried in a polymer matrix, sliced using anultra-microtome, and etched with Ar plasma.

A close scrutiny and comparison of FIG. 2, FIG. 3(A), and FIG. 3(B)indicates that the graphene-like layers in a HOGF are substantiallyoriented parallel to one another; but this is not the case for flexiblegraphite foil and graphene oxide paper. The inclination angles betweentwo identifiable layers in the highly conducting graphitic film aregenerally less than 10 degrees and mostly less than 5 degrees. Incontrast, there are so many folded graphite flakes, kinks, andmis-orientations in flexible graphite that many of the angles betweentwo graphite flakes are greater than 10 degrees, some as high as 45degrees (FIG. 2). Although not nearly as bad, the mis-orientationsbetween graphene platelets in NGP paper (FIG. 3(B)) are also high andthere are many gaps between platelets. The HOGF entity is essentiallygap-free.

FIG. 4(A) shows the thermal conductivity values of the HA/GO-derivedfilm, GO-derived film, HA suspension-derived HOGF, and flexible graphite(FG) foil, respectively, all plotted as a function of the final HTT.These data have clearly demonstrated the superiority of the presentlyinvented HA/GO-derived HOGF structures in terms of the achievablethermal conductivity at a given heat treatment temperature.

-   1) The HA/GO liquid crystal suspension-derived HOGF appears to be    superior to the GO gel-derived HOGF in thermal conductivity at    comparable final heat treatment temperatures. The heavy oxidation of    graphene sheets in GO gel might have resulted in high defect    populations on graphene surfaces even after thermal reduction and    re-graphitization. However, the presence of HA molecules seem to be    capable of helping to heal the defects or bridging the gaps between    GO sheets.-   2) Although the highly oriented films derived from HA alone exhibit    thermal conductivity values slightly lower than those derived from    GO alone, the HA, as a material, is naturally abundant and it does    not require the use of undesirable chemicals to produce HA. HA is    one order of magnitude less expensive than natural graphite (a raw    material for GO) and 2-4 orders of magnitude less expensive than GO.-   3) For comparison, we have also obtained conventional highly    oriented pyrolytic graphite (HOPG) samples from the polyimide (PI)    carbonization route. The polyimide films were carbonized at 500° C.    for 1 hour, at 1,000° C. for 3 hours, and at 1,500° C. for 12 hours    in an inert atmosphere. The carbonized PI films were then    graphitized at a temperature in the range of 2,500-3,000° C., under    a compressive force, for 1 to 5 hours to form a conventional HOPG    structure.

FIG. 4(B) shows the thermal conductivity values of the HA/GOsuspension-derived HOGF, the HA suspension-derived HOGF, and thepolyimide-derived HOPG, all plotted as a function of the final heattreatment temperature. These data show that the conventional HOPG,produced by using the carbonized polyimide (PI) route, exhibits aconsistently lower thermal conductivity as compared to the HA/GO-derivedHOGF, given the same HTT for the same length of heat treatment time. Forinstance, the HOPG from PI exhibits a thermal conductivity of 820 W/mKafter a graphitization treatment at 2,000° C. for 1 hour. At the samefinal graphitization temperature, the HA/GO-derived HOGF exhibits athermal conductivity value of 1,586 W/mK. It may be noted that PI isalso orders of magnitude more expensive than HA and the production of PIinvolves the use of several environmentally undesirable organicsolvents.

-   4) These observations have demonstrated a clear and significant    advantage of using the HA/GO or HA suspension approach to producing    HOGF versus the conventional PG approach to producing oriented    graphite crystals. As a matter of fact, no matter how long the    graphitization time is for the HOPG, the thermal conductivity is    always lower than that of a HA/GO liquid crystal-derived HOGF. It is    also surprising to discover that humic acid molecules are capable of    chemically linking with one another to form strong and highly    conducting graphitic films. It is clear that, the highly oriented HA    film (including highly oriented HA/GO film), and the subsequently    heat-treated versions are fundamentally different and patently    distinct from the flexible graphite (FG) foil, graphene/GO/RGO    paper/membrane, and pyrolytic graphite (PG) in terms of chemical    composition, crystal and defect structure, crystal orientation,    morphology, process of production, and properties.-   5) The above conclusion is further supported by the data in FIG.    4(C) showing the electric conductivity values of the HA/GO    suspension-derived and HA suspension-derived HOGF HOGF are far    superior to those of the FG foil sheets over the entire range of    final HTTs investigated.

Example 8: The Effect of Graphene Addition on the Properties of HA-BasedHighly Oriented Graphitic Films and Graphitic Films Derived Therefrom

Various amounts of graphene oxide (GO) sheets were added to HAsuspensions to obtain mixture suspensions wherein HA and GO sheets aredispersed in a liquid medium. The same procedure as described above wasthen followed to produce HOGF samples of various GO proportions. Thethermal conductivity data of these samples are summarized in FIG. 6,which indicate that the thermal conductivity values of the HOGF producedfrom the HA-GO mixtures are higher than those of the HOGF films producedfrom single-component alone.

Further surprisingly, there are synergistic effects that can be observedwhen both the HA sheets and GO sheets co-exist in proper proportions. Itseems that HA can help GO sheets (known to be highly defected) heal fromtheir otherwise defected structure. It is also possible that HAmolecules, being significantly smaller in size than GO sheets/molecules,can fill in the gaps between GO molecules and react therewith to bridgethe gaps. These two factors likely lead to the significantly improvedconductivity.

Examples 9: Tensile Strength of Various Graphene Oxide-Derived HOHAFilms

A series of HA/GO dispersion-derived HOGF, GO dispersion-derived HOGF,and HA-derived HOGF films were prepared by using a comparable final heattreatment temperature for all materials. A universal testing machine wasused to determine the tensile properties of these materials. The tensilestrength and modulus of these various samples prepared over a range ofheat treatment temperatures are shown in FIG. 7(A) and FIG. 7(B),respectively. For comparison, some tensile strength data of RGO paperand flexible graphite foil are also summarized in FIG. 7(A).

These data have demonstrated that the tensile strength of the graphitefoil-derived sheets increases slightly with the final heat treatmenttemperature (from 14 to 29 MPa) and that of the GO paper(compressed/heated sheets of GO paper) increases from 23 to 52 MPa whenthe final heat treatment temperature increases from 700 to 2,800° C. Incontrast, the tensile strength of the HA-derived HOGF increasessignificantly from 28 to 93 MPa over the same range of heat treatmenttemperatures. Most dramatically, the tensile strength of the HA/GOsuspension-derived HOGF increases significantly from 32 to 126 MPa. Thisresult is quite striking and further reflects the notion that the HA/GOand HA dispersion contains highly oriented/aligned, chemically activeHA/GO and HA sheets/molecules that are capable of chemical linking andmerging with one another during the heat treatment, while the grapheneplatelets in the conventional GO paper and the graphite flakes in the FGfoil are essentially dead platelets. The HA or HA/GO-based highlyoriented films and the subsequently produced graphitic films is a newclass of material by itself.

As a point of reference, the film, obtained by simply sprayingHA-solvent solution onto a glass surface and drying the solvent, doesnot have any strength (it is so fragile that you can break the film bysimply touch the film with a finger). After heat treating at atemperature >100° C., this film became fragmented (broken into a hugenumber of pieces). In contrast, the highly oriented HA film (wherein allHA molecules or sheets are highly oriented and packed together), uponheat treatment at 150° C. for one hour, became a film of good structuralintegrity, having a tensile strength >24 MPa.

Example 10: The Novel Effect of Metal Foil on Heat-Induced ChemicalLinking of Humic Acid Molecules

Shown in FIG. 8 are the thermal conductivity values of three HA-derivedhighly oriented films. The first one was obtained by heat-treating a HAfilm peeled off from a glass surface. The second one was coated on Tisurface and the film was bonded to Ti surface during the heat-treatment.The third one was coated on a Cu foil surface and was bonded to the Cufoil surface during the heat treatment. With the same final heattreatment temperatures, the metal foil-supported HA films exhibitsignificantly higher thermal conductivity values as compared to those ofthe films peeled off from PET film surface prior to heat treating. TheCu and Ti foil appear to be capable of providing some kind of catalyticeffect on the heat-induced chemical linking or merging between humicacid molecules in intimate contact with Cu or Ti. This is trulyunexpected. Even more surprising is the discovery that the differencesin conductivity are very large. Furthermore, when supported on Cu foil,a HA film after a heat treatment at 1,250° C. exhibits a thermalconductivity of 1,432 W/mK. The same value was achieved with theHA-derived film after a heat treatment at 2,500° C. for the same lengthof time without the benefit of being catalyzed by Cu or Ti.

Example 11: Li—S Cell Containing a Humic Acid-Bonded Metal Foil CurrentCollector at the Anode and at the Cathode

Three (3) Li—S cells were prepared and tested, each one having a lithiumfoil as the anode active material, a sulfur/expanded graphite composite(75/25 wt. ratio) as the cathode active material, 1M of LiN(CF₃SO₂)₂ inDOL as the electrolyte, and a Celgard 2400 as the separator. The firstcell (a baseline cell for comparison) contains a 10-μm thick Cu foil asthe anode current collector and a 20-μm thick Al foil as the cathodecurrent collector. The second cell (another baseline cell forcomparison) has a 10-μm thick GO-resin layer as the anode currentcollector and a sheet of 14-μm RGO-coated Al foil as the cathode currentcollector. The third cell has a HA-bonded Cu foil (totally 12-μm thick)of the present invention as the anode current collector and a sheet of a20-μm thick HA-coated Al foil as the cathode current collector.

Charge storage capacities were measured periodically and recorded as afunction of the number of cycles. The specific discharge capacity hereinreferred to is the total charge inserted into the cathode during thedischarge, per unit mass of the composite cathode (counting the weightsof cathode active material, conductive additive or support, and thebinder, but excluding the current collectors). The specific energy andspecific power values presented in this section are based on the totalcell weight (including anode and cathode, separator and electrolyte,current collectors, and packaging materials). The morphological ormicro-structural changes of selected samples after a desired number ofrepeated charging and recharging cycles were observed using bothtransmission electron microscopy (TEM) and scanning electron microscopy(SEM).

FIG. 9(A) shows the discharge capacity values of the three cells each asa function of the charge/discharge cycle number. Each cell was designedto have an initial cell capacity of 100 mAh to facilitate comparison. Itis clear that the Li—S cell featuring the presently invented HA-bondedcurrent collector at both the anode and the cathode exhibits the moststable cycling behavior, experiencing a capacity loss of 6% after 50cycles. The cell containing GO/resin-coated Cu and GO-coated Al currentcollector suffers from a 23% capacity decay after 50 cycles. The cellcontaining a Cu foil anode current collector and an Al foil cathodecurrent collector suffers from a 26% capacity decay after 50 cycles.Post-cycling inspection of the cells indicate that the Al foil in allprior art electrodes suffered a severe corrosion problem. In contrast,the presently invented humic acid oxide-bonded Al current collectorsremain intact.

FIG. 9(B) shows the Ragone plots (gravimetric power density vs.gravimetric energy density) of the three cells. It is of interest tonote that our HA-bonded metal foil current collectors surprisinglyimpart both higher energy density and higher power power density to theLi—S cell compared to prior art graphene/resin-coated current collectorat the anode (with GO-coated Al foil at the cathode), and Cu/Al currentcollectors. This is quite unexpected considering that Cu foil has anelectrical conductivity that is more than one order of magnitude higherthan that of the graphene film and HA film.

Example 12: Magnesium-Ion Cell Containing a HA-enabled Current Collectorat the Anode and at the Cathode

For the preparation of a cathode active material (Magnesium ManganeseSilicate, Mg_(1.03)Mn_(0.97)SiO₄), reagent-grade KCl (melting point=780°C.) was used as flux after drying for 3 h at 150° C. under vacuum. Thestarting materials were magnesium oxide (MgO), manganese (II) carbonate(MnCO₃) and silicon dioxide (SiO₂, 15-20 nm) powder. The stoichiometricamounts for the precursor compounds were controlled with the molar ratioof 1.03:0.97:1 for Mg:Mn:Si. The mixture (flux/reactants molar ratio=4)was hand-ground in a mortar by pestle for a 10 minutes, and then pouredinto a corundum crucible. Then, the powder mixture was dried at 120° C.for 5 h in a vacuum to minimize the water content in the mixture.Subsequently, the mixture was immediately transferred to a tube furnaceand heated in a reductive atmosphere (Ar+5 wt % H2) at 350° C. for 2 hto remove carbonate groups. This was followed by final firing at varioustemperatures at a rate of 2° C./min for 6 h, then cooling to roomtemperature naturally. Finally, the product (Magnesium ManganeseSilicate, Mg_(1.03)Mn_(0.97)SiO₄) was washed three times with deionizedwater to dissolve any remaining salt, separated by centrifugation, anddried under vacuum at 100° C. for 2 h.

The electrodes (either the anode or cathode) were typically prepared bymixing 85 wt % of an electrode active material (e.g.Mg_(1.03)Mn_(0.97)SiO₄ particles, 7 wt % acetylene black (Super-P), and8 wt % polyvinylidene fluoride binder (PVDF, 5 wt % solid contentdissolved in N-methyl-2-pyrrolidinoe (NMP)) to form a slurry-likemixture. After coating the slurry on an intended current collector, theresulting electrode was dried at 120° C. in vacuum for 2 h to remove thesolvent before pressing. Three cells having different current collectorswere investigated: first cell having HA-bonded Cu foil and HA-bonded Alfoil as the anode and cathode current collectors, respectively; secondcell having GO/resin-coated Cu foil and GO-coated Al foil (nopre-etching) as the anode and cathode current collector, respectively (aprior art cell); third cell having a Cu foil anode current collector andAl foil cathode current collector (a prior art cell).

Subsequently, the electrodes were cut into disks (diameter=12 mm) foruse as a cathode. A thin sheet of magnesium foil was attached to theanode current collector surface, and a piece of porous separator (e.g.,Celgard 2400 membrane) was, in turn, stacked on top of the magnesiumfoil. A piece of cathode disc coated on a cathode current collector wasused as a cathode and stacked over the separator layer to form a CR2032coin-type cell. The electrolyte used was 1 M of Mg(AlCl₂EtBu)₂ in THF.The cell assembly was performed in an argon-filled glove-box. The CVmeasurements were carried out using a CHI-6 electrochemical workstationat a scanning rate of 1 mV/s. The electrochemical performance of thecells was also evaluated by galvanostatic charge/discharge cycling at acurrent density of from 50 mA/g to 10 A/g (up to 100 A/g for somecells), using an Arbin and/or a LAND electrochemical workstation.

FIG. 10 shows the cell discharge specific capacity values of the threecells each as a function of the charge/discharge cycle number. It isclear that the Mg-ion cell featuring the presently invented currentcollectors at both the anode and the cathode exhibits the most stablecycling behavior, experiencing a capacity loss of 2.5% after 25 cycles.The cell containing GO/resin-coated Cu foil and GO-coated Al foilcurrent collectors suffers from a 17% capacity decay after 25 cycles.The cell containing a Cu foil anode current collector and an Al foilcathode current collector suffers from a 30% capacity decay after 25cycles. Post-cycling inspection of the cells indicate thatGO/resin-coated Cu foil and GO-coated Al foil current collectors gotswollen and showed some delamination from the cathode layer and that Alfoil suffered a severe corrosion problem. In contrast, inventiveHA-bonded metal foil current collectors remain intact.

Example 13: Chemical and Mechanical Compatibility Testing of VariousCurrent Collectors for Various Intended Batteries or Supercapacitors

As demonstrated in Examples 11 and 12 above, long-term stability of acurrent collector relative to the electrolyte of a battery orsupercapacitor is a major concern. In order to understand the chemicalstability of various current collectors, a major task was undertaken toexpose current collectors in several representative electrolytes. Afteran extended period of time (e.g. 30 days), the current collector wasremoved from the electrolyte solution and observed using optical andscanning electron microscopy (SEM). The results are summarized in Table3 below, which consistently demonstrate that the inventive HA-bondedmetal foil current collectors are highly compatible with all kinds ofliquid electrolytes commonly used in batteries and supercapacitors. Theinventive materials are resistant to any chemical attack. TheseHA-protected current collectors are essentially electrochemically inertover a voltage range of 0-5.5 volts Vs. Li/Li⁺, suitable for use withjust about any battery/capacitor electrolyte.

It may be noted that each current collector must be connected to a tabthat is, in turn, connected to an external circuit wire. The currentcollector must be mechanically compatible with the tab, being readily oreasily fastened or bonded thereto. We have found that CVD graphene filmsjust cannot be mechanically fastened to the tab without being easilybroken or fractured. Even with the assistance of adhesive, the CVD filmis easily fractured during the procedures of connecting to a tab orbattery cell packaging.

TABLE 3 Results of current collector-electrolyte compatibility testing.Sample Intended battery Current No. or supercapacitor collectorElectrolyte Observations Li-1A Li-ion or Li HA film- 1M LiPF₆ in Remainsintact, no swelling, metal bonded Cu foil PC + DME no micro-cracking; nopits. Li-1B Li-ion or Li CVD graphene 1M LiPF₆ in Micro-cracks formedalong metal film PC + DME grain boundaries Li-1C Li-ion or Li RGO coatedon 1M LiPF₆ in RGO layer swollen, metal PET film PC + DME delaminationfrom PET film Na-1A Na-ion or Na HA film- 1M NaClO₄ in Remains intact,no swelling, metal bonded Ti foil DOL + DEC no micro-cracking Na-1BNa-ion or Na CVD graphene 1M NaClO₄ in Micro-cracks formed along metalfilm DOL + DEC grain boundaries Sup-1A Supercapacitor HA film- 1M H₂SO₄in Remains intact, no swelling, or hybrid bonded Al foil water nomicro-cracking Sup-1B Supercapacitor Flexible 1M H₂SO₄ in Severelyswollen, flaking or hybrid graphite foil water (graphite flakes comingoff) Sup-1C Supercapacitor HA film- Alkylammonium Remains intact, noswelling, or hybrid bonded Al foil in acetonitrile no micro-cracking, nopitting corrosion Sup-1D Supercapacitor Carbon-coated AlkylammoniumCorrosion of Al layer; some or hybrid Al in acetonitrile carbon flakingZn-1A Zinc-air HA film- KOH in water Remains intact, no swelling, bondedNi foil no micro-cracking Zn-1B Zinc-air Flexible KOH in water Severelyswollen, flaking graphite foil

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of highly conducting material:humic acid-derived thin film-bonded on metal foil surface(s). Thechemical composition, structure (crystal perfection, grain size, defectpopulation, etc), crystal orientation, morphology, process ofproduction, and properties of this new class of materials arefundamentally different and patently distinct from flexible graphitefoil, polymer-derived pyrolytic graphite, CVD-derived PG (includingHOPG), and catalytic CVD graphene thin film that are free-standing orcoated on a metal foil. The thermal conductivity, electricalconductivity, scratch resistance, surface hardness, and tensile strengthexhibited by the presently invented materials are much higher than whatprior art flexible graphite sheets, paper of discrete graphene/GO/RGOplatelets, or other graphitic films could possibly achieve. TheseHA-derived thin film structures have the best combination of excellentelectrical conductivity, thermal conductivity, mechanical strength,surface scratch resistance, hardness, and no tendency to flake off.

The invention claimed is:
 1. A process for producing a highly orientedgraphitic film-bonded metal foil, said process comprising: (a) preparinga dispersion of humic acid (HA) sheets dispersed in a liquid medium,wherein said HA sheets contain an oxygen content higher than 5% byweight; (b) dispensing and depositing said HA dispersion onto at leastone primary surface of a metal foil to form a wet layer of HA, whereinsaid dispensing and depositing procedure includes subjecting saiddispersion to an orientation-inducing stress; (c) partially orcompletely removing said liquid medium from the wet layer of HA to forma dried HA layer having hexagonal carbon planes and an inter-planarspacing d₀₀₂ of 0.4 nm to 1.3 nm as determined by X-ray diffraction; (d)heat-treating said dried HA layer at a first heat treatment temperaturehigher than 80° C.; (e) further heat-treating said humic acidfilm-bonded metal foil at a second heat treatment temperature higherthan the first heat treatment temperature for a sufficient period oftime to produce a graphitic film-bonded metal foil current collector,wherein said graphitic film has an oxygen content or non-carbon elementcontent less than 5% by weight, a degree of graphitization no less than80% and/or a mosaic spread value less than 0.4; and (f) compressing saidgraphitic film to produce a highly conducting, highly oriented graphiticfilm having a physical density from 1.3 g/cm³ to 2.2 g/cm³, comprisinginter-connected, merged or thermally reduced sheets that aresubstantially parallel to each other and are chemically bonded andparallel to at least one primary surface, and having a thermalconductivity from 500 W/mK to 1,500 W/mK, and/or an electricalconductivity from 1,000 S/cm to 12,000 S/cm.
 2. The process of claim 1,wherein said HA dispersion further contains graphene sheets or moleculesdispersed therein and said HA-to-graphene ratio is from 1/100 to 100/1and said graphene is selected from pristine graphene, graphene oxide,reduced graphene oxide, graphene fluoride, graphene bromide, grapheneiodide, boron-doped graphene, nitrogen-doped graphene, chemicallyfunctionalized graphene, or a combination thereof.
 3. The process ofclaim 2, wherein said graphene sheets contain chemically functionalizedgraphene containing a chemical functional group selected from a polymer,SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′,SR′, SiR′₃, Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X,TlZ₂ and Mg—X; wherein y is an integer equal to or less than 3, R′ ishydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, orpoly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl,fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate ortrifluoroacetate, or a combination thereof.
 4. The process of claim 1,wherein said HA are in an amount sufficient to form a liquid crystalphase in said liquid medium.
 5. The process of claim 1, wherein saiddispersion contains a first volume fraction of HA dispersed in saidliquid medium that exceeds a critical volume fraction (V_(c)) for aliquid crystal phase formation and said dispersion is concentrated toreach a second volume fraction of HA, greater than the first volumefraction, to improve a HA sheet orientation.
 6. The process of claim 5,wherein said first volume fraction is equivalent to a weight fraction offrom 0.05% to 3.0% by weight of HA in said dispersion.
 7. The process ofclaim 5, wherein said dispersion is concentrated to contain higher than3.0% but less than 15% by weight of HA dispersed in said liquid mediumprior to said step (b).
 8. The process of claim 1, wherein saiddispersion further contains a polymer dissolved in said liquid medium orattached to said HA.
 9. The process of claim 1, wherein said second heattreatment temperature is higher than 1,500° C. for a length of timesufficient for decreasing an inter-plane spacing d₀₀₂ to a value lessthan 0.36 nm and decreasing the oxygen content or non-carbon elementcontent to less than 0.1% by weight.
 10. The process of claim 1, whereinsaid liquid medium consists of water or a mixture of water and analcohol.
 11. The process of claim 1, wherein said liquid medium containsa non-aqueous solvent selected from polyethylene glycol, ethyleneglycol, propylene glycol, an alcohol, a sugar alcohol, a polyglycerol, aglycol ether, an amine based solvent, an amide based solvent, analkylene carbonate, an organic acid, or an inorganic acid.
 12. Theprocess of claim 1, which is a roll-to-roll process wherein said step(b) includes feeding a sheet of said metal foil from a roller to adeposition zone, depositing a layer of HA dispersion onto at least oneprimary surface of said sheet of metal foil to form said wet layer of HAdispersion thereon, drying said HA dispersion to form the dried HA layerdeposited on metal foil surface, and collecting said HA layer-depositedmetal foil sheet on a collector roller.
 13. The process of claim 1,wherein said first heat treatment temperature contains a temperature inthe range of 100° C.-1,500° C. and the highly oriented humic acid filmhas an oxygen content less than 2.0%, an inter-planar spacing less than0.35 nm, a physical density from 1.6 g/cm³ to 2.2 g/cm³, a thermalconductivity from 800 W/mK to 1500 W/mK, and/or an electricalconductivity from 2,500 S/cm to 12,000 S/cm.
 14. The process of claim 1,wherein said first heat treatment temperature contains a temperature inthe range of 1,500° C.-2,100° C. and the highly oriented humic acid filmhas an oxygen content less than 1.0%, an inter-planar spacing less than0.345 nm, a thermal conductivity from 1,000 W/mK to 1500 W/mK, and/or anelectrical conductivity from 5,000 S/cm to 12,000 S/cm.
 15. The processof claim 1, wherein said first and/or second heat treatment temperaturecontains a temperature greater than 2,100° C. and the highly conductinggraphitic film has an oxygen content no greater than 0.1%, aninter-graphene spacing less than 0.340 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 1,300 W/mK, and/oran electrical conductivity no less than 8,000 S/cm.
 16. The process ofclaim 1, wherein said second heat treatment temperature contains atemperature no less than 2,500° C. and the highly conducting graphiticfilm has an inter-graphene spacing less than 0.336 nm, a mosaic spreadvalue no greater than 0.4, and/or an electrical conductivity greaterthan 10,000 S/cm.
 17. The process of claim 1, wherein said HA sheetshave a maximum original length and said highly oriented humic acid filmcontains HA sheets having a length larger than said maximum originallength.
 18. The process of claim 1, wherein said highly conductinggraphitic film is a poly-crystal graphene structure having a preferredcrystalline orientation as determined by said X-ray diffraction method.19. The process of claim 1, wherein said step (e) of heat-treatinginduces chemical linking, merging, or chemical bonding of HA sheets withother HA sheets, or with graphene sheets to form a graphitic structure.20. The process of claim 1, wherein said highly oriented graphitic filmhas a tensile strength greater than 80 MPa, and/or an elastic modulusgreater than 60 GPa.
 21. The process of claim 1, wherein said highlyoriented graphitic film has a tensile strength greater than 120 MPa,and/or an elastic modulus greater than 120 GPa.
 22. A process forproducing a highly oriented humic acid film-bonded metal foil currentcollector for use in a battery or supercapacitor, said processcomprising: (a) preparing a dispersion of chemically functionalizedhumic acid sheets in a liquid medium, wherein said chemicallyfunctionalized humic acid sheets contain a non-carbon element contenthigher than 5% by weight; (b) dispensing and depositing said dispersiononto at least one primary surface of a metal foil to form a wet layer ofchemically functionalized humic acid, wherein said dispensing anddepositing procedure includes subjecting said dispersion to anorientation-inducing stress; (c) partially or completely removing saidliquid medium from the wet layer of chemically functionalized humic acidto form a dried chemically functionalized humic acid layer havinghexagonal carbon planes and an inter-planar spacing d₀₀₂ of 0.4 nm to1.3 nm as determined by X-ray diffraction; and (d) heat-treating saiddried chemically functionalized humic acid layer at a first heattreatment temperature higher than 80° C. for a sufficient period of timeto produce said highly oriented chemically functionalized humic acidfilm-bonded metal foil wherein said highly oriented chemicallyfunctionalized humic acid film contains inter-connected, merged orthermally reduced chemically functionalized humic acid sheets that aresubstantially parallel to each other and are chemically bonded andparallel to said at least one primary surface and said chemicallyfunctionalized humic acid sheets have a physical density from 1.3 g/cm³to 2.2 g/cm², a thermal conductivity from 250 W/mK to 1500 W/mK and/oran electrical conductivity from 500 S/cm to 12,000 S/cm; (e) optionallyheat-treating said highly oriented chemically functionalized humic acidfilm at a second heat treatment temperature higher than the first heattreatment temperature; and (f) optionally compressing said highlyoriented chemically functionalized humic acid film.
 23. The process ofclaim 22, wherein said chemically functionalized humic acid contains achemical functional group selected from a polymer, SO₃H, COOH, NH₂, OH,R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃,Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂ andMg—X; wherein y is an integer equal to or less than 3, R′ is hydrogen,alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl orcycloaryl, X is halide, and Z is carboxylate or trifluoroacetate, or acombination thereof.
 24. The process of claim 22, wherein highlyoriented chemically functionalized humic acid film exhibits a degree ofgraphitization no less than 80% and/or a mosaic spread value less than0.4.
 25. The process of claim 22, wherein said highly orientedchemically functionalized humic acid film has an electrical conductivitygreater than 8,000 S/cm, a thermal conductivity greater than 1,200 W/mK,a physical density greater than 2.0 g/cm³, a tensile strength greaterthan 100 MPa, and/or an elastic modulus greater than 80 GPa.
 26. Theprocess of claim 22, wherein said chemically functionalized humic aciddispersion further comprises graphene sheets or molecules dispersedtherein and said chemically functionalized humic acid-to-graphene ratiois from 1/100 to 100/1 and said graphene is selected from pristinegraphene, graphene oxide, reduced graphene oxide, graphene fluoride,graphene bromide, graphene iodide, boron-doped graphene, nitrogen-dopedgraphene, chemically functionalized graphene, and combinations thereof.